Loaded with chocolate chips and M&Ms, Monster Cookie Bars are thick & chewy, and oh-so-good! These peanut butter oatmeal cookie bars can easily be adapted into a festive treat for any holiday, from the 4th of July to Christmas – and did I mention you only need one bowl?!

Four monster cookie bars stacked on each other

Irresistible Peanut Butter Oatmeal Cookie Bars

Everyone needs an easy, irresistible cookie bars recipe in their back pocket, dontcha think? Because a pan of chewy Monster Cookie Bars could not be more perfect for a picnic at the park, a backyard bbq, or a neighborhood block party.

This is a family recipe, from Blake’s cousin Melanie. And they are PERFECTION. They’re thick and chewy and moist, with just the right amount of peanut butter and oats. And they’re loaded with chocolate chips and M&M’s. These bars make people HAPPY!

This easy bars recipe is super quick to make (yay for you!) and always a hit with guests (yay for them!).

Another favorite cookie bars recipe for you >>> everyone loves these chewy chocolate chip cookie bars!

Overhead view of monster cookie bars in the pan

A hand picking up the top cookie bar from a stack of monster cookie bars

What Are Monster Cookie Bars?

Monster cookie bars are the cookie bar version of monster cookies. Essentially, monster cookie dough is spread into a rimmed pan, baked in one big slab, and then cut into individual bars. Cookie bars are easier and quicker to make than individual cookies, while still giving you the same awesome flavor!

Monster cookies are thick, chewy peanut butter & oatmeal cookies that are loaded up with chocolate and other mix-ins. In this version, we keep it classic with lots of M&Ms and chocolate chips but I’ve included some other possibilities as well.

Basically, they are THE cookie to make if you can’t choose between peanut butter, oatmeal, or chocolate chip!

Overhead view of monster cookie bar ingredients

What You’ll Need

Pantry staples, to include peanut butter, oats, and lots of chocolate, are all you need for these peanut butter oatmeal cookie bars.

  • Butter – Room temperature softened butter will get everything incorporated best.
  • Creamy peanut butter – I use Skippy brand, our family’s favorite PB.
  • Sugar – Use equal parts brown sugar & granulated sugar to make these cookie bars.
  • Eggs
  • Vanilla extract
  • All-purpose flour
  • Old fashioned oats – Regular old-fashioned oats work best, not quick oats or steel-cut oats.
  • Baking soda
  • Semi-sweet chocolate chips – I always use Guittard brand but use whatever you like.
  • M&Ms – Use regular colors or a special bag of festive colors for whatever holiday you’re celebrating. (See below for ideas).
  • Chocolate frosting and sprinkles – Optional, for decorating.
Patriotic monster cookie bars in a pan
Overhead view of a platter of patriotic cookie bars
Patriotic cookie bars topped with chocolate frosting and sprinkles

Festive Variations

When I first made these cookie bars, I wanted to add a little patriotic fun for a 4th of July gathering. So I swapped the regular M&Ms for a bag of red, white, and blue M&Ms plus a swirl of fudgy frosting and some patriotic sprinkles.

The end result was eye-catching and I definitely recommend customizing these bars for whatever holiday – it’s so easy! Just change the M&M color!

And while the added frosting & sprinkles is an extra fun touch, it’s still a matter of your own choice. Add it. Or don’t! The cookie bars are showy enough with the custom colored candies. Keep it simple…or go crazy with it!

Just know, whether you keep them basic or dress them up, expect these monster cookie bars to disappear FAST!

Here are a few more variation ideas. This is such a great recipe for celebrating different holidays and events!

  • Patriotic Cookie Bars. See the photos in this post, where I used red, white & blue M&Ms. I also added a swirl of fudgy frosting + red, white & blue sprinkles!
  • Christmas Cookie Bars. For a festive holiday treat, use a bag of red and green M&Ms.
  • Valentine’s Day Cookie Bars. So fun when you mix in a bag of red, pink & white M&Ms!
  • Halloween Cookie Bars. Similarly, you can find a bag of Halloween-colored M&Ms as well. Reese’s pieces are also a great addition to Halloween cookie bars!
  • Easter Cookie Bars. For Easter or just general spring cookie bars, grab a bag of pastel M&Ms.
  • Graduation Cookie Bars. This is a fun treat for graduation parties, whether you’re celebrating graduation from kindergarten, high school, or college. Use M&Ms that are specific to the school colors.
Overhead view monster cookie bars topped with chocolate icing

Other Mix-In Ideas

Monster cookies are a bit like kitchen sink cookies, so feel free to go above and beyond the M&Ms and semi-sweet chocolate chips in this recipe. Here are a few ideas for different add-ins:

  • Dark chocolate chips
  • White chocolate chips
  • Butterscotch chips
  • Peanut butter chips… or any other flavored chips!
  • Chocolate chunks instead of chips
  • Crushed pretzels
  • Chopped candy
  • Raisins – You’ll commonly find raisins in monster cookies, but my family always requests “no raisins”. Feel free to mix some in if you like!

How to Make Monster Cookie Bars

Here’s how to make these chewy, chocolatey monster cookie bars.

  • Make the dough. Cream together the butter, peanut butter, and sugars. Add in the eggs, vanilla, and mix to incorporate. Add the remaining dry ingredients – flour, oats, baking soda, and salt. Mix. Reserve 1/2 a cup of the chocolate chips and M&Ms then fold the remaining amounts into the dough.
  • Bake. Transfer the dough to the greased pan. Lightly press the remaining chocolate chips and M&Ms over the top. Bake for 14 to 15 minutes. Take care not to overbake, as these are meant to be chewy. Let cool on a wire rack before cutting into individual bars.
Four monster cookie bars stacked on each other
Angled views of monster cookie bars topped with icing

How to Cut Perfect Slices

Most of the time when I make these peanut butter oat cookie bars, I’m not too preoccupied with the perfect cut. After all, they’re meant to be a casual treat. And they’re so delicious that a little rough edge is surely not going to detract from how much anyone loves them!

But if I’m serving them for a special gathering, a clean-cut does go a long way towards presentation. To achieve clean cuts in these bars, as well as other brownies and cakes, I’m a big fan of lining the pan with foil or parchment paper and letting it overhang on the long sides of the pan.

This makes it so much easier to remove the bars from the pan, for easier cutting. It also keeps the outside edges nice and clean. As far as cutting, if you have a rigid pastry cutter, it does double-duty as a great bar-cutting tool. Otherwise, I use a sharp knife and wipe it after each cut.

A hand grabbing a monster cookie bar

Decorating & Serving

For serving, I often just cut the squares and let guests take bars straight from the pan with a little spatula. Or I place the cut bars on a platter.

However when I decorate them for special events and holidays, I do line the pan with parchment paper so I can make clean cuts for a nicer presentation. Sometimes I add some frosting and sprinkles, other times I don’t.

My chocolate buttercream would be perfectly delicious swirled on top – or simply use a can of fudge frosting from the baking aisle of the grocery store. Vanilla frosting would also work if you want a little less chocolate and a splash of white color.

Transfer the frosting to a pastry bag with a medium star tip and swirl the frosting onto each bar. Add some sprinkles of choice and you have a sweet and festive dessert ready to go!

Angled view of monster cookie bars with frosting

How to Store

Monster cookie bars will keep well for up to 4 days stored at room temperature – just keep them in an airtight container. Since there’s so much chocolate, I’d recommend keeping them somewhere cool and out of direct sunlight. And as per the usual for baked treats, these are best enjoyed during the first day or two.

They can also be frozen for up to 3 months. Place them in a freezer-safe bag or container, separating layers with parchment or wax paper. Thaw on the counter and enjoy.

More Cookie Recipes:

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Four monster cookie bars stacked on each other

Monster Cookie Bars

Yield: 48 bars
prep time: 15 minutes
cook time: 15 minutes
total time: 30 minutes
Loaded with chocolate chips and M&Ms, Monster Cookie Bars are thick & chewy, and oh-so-good! These peanut butter oatmeal cookie bars can easily be adapted into a festive treat for any holiday, from the 4th of July to Christmas – and did I mention you only need one bowl?!
4.6 Stars (7 Reviews)
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Ingredients

  • ½ cup + 1 tablespoon unsalted butter at room temperature, divided
  • cups creamy peanut butter
  • 1 cup packed brown sugar
  • 1 cup granulated sugar
  • 3 large eggs
  • 1 teaspoon pure vanilla extract
  • 1 cup all-purpose flour
  • cups old fashioned oats
  • 2 teaspoons baking soda
  • ½ teaspoon kosher salt
  • 12 ounces semi-sweet chocolate chips – I always use Guittard brand
  • 1 12.6-ounce bag M&M’s – customize with red, white & blue for July 4th or other colors for other holidays and occasions
  • chocolate frosting and sprinkles optional – see note below

Instructions

  • Preheat oven to 350° F. Lightly butter a 12'' x 17'' jelly roll pan with 1 tablespoon of the butter. Set aside.
  • In the bowl of a stand mixer, cream together remaining 1/2 cup butter, peanut butter, brown sugar, and sugar. Add eggs and vanilla, and mix until incorporated. Add flour, oats, baking soda, and salt, and mix again until incorporated.
  • Reserve about 1/2 cup each of the chocolate chips and M&M's, and then fold the remaining into the batter with a spatula. Press the batter evenly into prepared pan. Sprinkle the reserved chocolate chips and M&M's over the top, pressing them in slightly. Bake for 14 to 15 minutes. These bars are meant to be chewy; do not over bake! Remove from oven and let pan cool completely on wire rack. Then cut into individual bars.

Notes

These bars are fabulous just the way they are, but if you want to add a little more festivity for the 4th of July, add some chocolate (or white) frosting and sprinkles! My favorite chocolate buttercream would be great, or simply use a can of fudge frosting from the baking aisle at the grocery store. Place frosting in a pastry bag fitted with a medium star tip and swirl the frosting onto the top of each bar. Add some red, white, and blue sprinkles, and you're set for a celebration!
Adapted from the recipe box of Blake’s cousin, Melanie Sorenson.

Nutrition Information:

Serving: 1 Calories: 155kcal Carbohydrates: 21g Protein: 4g Fat: 7g Saturated Fat: 2g Polyunsaturated Fat: 4g Cholesterol: 12mg Sodium: 112mg Fiber: 2g Sugar: 13g
Nutrition information is automatically calculated by Spoonacular. I am not a nutritionist and cannot guarantee accuracy. If your health depends on nutrition information, please calculate again with your own favorite calculator.
Did you make this recipe?Please leave a comment below. And share a photo on Instagram with the hashtag #afarmgirlsdabbles or tag @farmgirlsdabble!

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Four monster cookie bars stacked on each other

I first shared this recipe here in 2013, and then updated the post in 2022.