Lightly spray a 12" x 17" pan with nonstick spray and set aside. Unwrap the butter and reserve the paper wrappers for pressing the final mixture into the pan. The wrappers provide a great nonstick means for pressing in that super sticky mixture. Measure out the cereal into a large bowl and set aside.

In a very large pot (I use a large soup pot) over medium heat, melt butter, chocolate chips, 40 ounces (four 10-ounce bags) of the marshmallows, and peanut butter. Stir almost continuously with a sturdy wooden spoon, scraping the bottom of the pot to make sure the mixture isn't scorching. The mixture will be very thick. When just about completely combined, remove pot from heat and stir vigorously to finish combining.

Now you want to move quickly in these last steps because as this thick mixture cools, it gets harder to fold. Have your cereal and remaining marshmallows ready to go! That wooden spoon is going to give you a good workout! Quickly fold in the cereal until completely mixed throughout. Then sprinkle the remaining 10 ounces of marshmallows over the top and quickly fold them in. Aim to keep the marshmallows intact, so there are extra-marshmallow bites scattered throughout the bars.

Transfer mixture to prepared pan and use the buttery side of the reserved butter wrappers to press the mixture evenly into the pan. Let cool completely and then cut into squares.