Set a rack in the center of the oven. Preheat oven to 350°. Spray two 9″ round cake pans with non-stick cooking spray and then lay 19 Oreos in an even layer on the bottom of each pan. Set pans aside.

In the bowl of a stand mixer, using a paddle attachment, blend the cocoa, sugar, cake flour, baking soda, and salt on a low speed for 30 seconds. Add eggs, sour cream, and vanilla and mix for 1 minute on medium-low speed. Remove bowl from the mixer. In a small bowl, whisk together hot coffee and chocolate until chocolate is fully melted. Add coffee/chocolate mixture to the stand mixer bowl and stir by hand with a spatula until all ingredients are fully incorporated.

Divide batter evenly between the two prepared pans, covering the Oreos completely. Bake for 25 to 32 minutes, or until toothpick inserted into the center comes out clean. Remove pans from oven and let cool on a wire rack for 10 minutes. Run a knife around the outer edge of each pan and carefully remove cakes from their pans, setting them on wire racks until they cool to room temperature.

In the bowl of a stand mixer, using the paddle attachment, mix butter and shortening until fully combined. Add the sugar, one cup at a time, alternating with the milk and vanilla. The filling should be very thick, but you can add more milk if you need to thin it a bit.

In the bowl of a stand mixer, using a paddle attachment, cream the butter, cocoa, and vanilla at medium speed for 1 to 2 minutes. Add the powdered sugar one cup at a time, adding in milk whenever the frosting gets too stiff or the mixer starts to struggle. After the last of the powdered sugar has been added, add the rest of the milk and mix until the frosting is smooth and silky, about 2 minutes. For a stiffer consistency, add more powdered sugar, 1/4 cup at a time. If the frosting is too stiff, add in more milk, 1 tablespoon at a time. I made a larger batch of chocolate buttercream and used a different technique to finish the cake than Amanda did (she covered hers in Oreo cookie crumbs), and would recommend adding just a little more milk to make the buttercream easier to work with in creating a finished frosted look.

Place one cooled layer on a cake stand or large platter, Oreo cookie side down. Using an offset spatula, cover the top with the Oreo filling, in an even layer. Top with the remaining layer of cake, Oreo cookie side up. Then frost the sides and top with the chocolate buttercream. You may or may not use all the chocolate buttercream, depending on your preference for frosting quantity. If you want the same look as in my photos, I used all but about 3/4 cup of the frosting. I also added a wide tube of frosting around the base of the cake using a large plain pastry tip, and then sprinkled the base with colorful sprinkles. The buttercream sets up pretty quickly, so add the sprinkles as soon as you get done with the frosting.

Amanda recommends freezing her finished cakes for at least 6 hours before serving, to ensure ease of slicing and preservation of the surprise-inside designs. Then use a sharp knife, dipped in a tall glass of hot water and dried, to cut your slices. Dip and dry the knife after each cut. Enjoy!!