From "Surprise Inside Cakes" by Amanda Rettke from I Am Baker. Reprinted with permission.
Ingredients
Chocolate Cake
3/4cupsunsweetened cocoa
1 1/2cupssugar
1 1/2cupscake flour
1tspbaking soda
Pinchof salt
2large eggs at room temperature
1cupsour creamat room temperature
2tspsgood quality vanilla extract
1/2cuphot coffee or hot water
1cupmilk chocolateabout 4 ounces finely chopped
1 15.25ouncepackage Oreos
Oreo Filling
1/4cup1/2 stick unsalted butter
1/4cupshortening
3cupspowdered sugar
2 to 3tablespoonsmilk
2tspsvanilla extract
Chocolate Buttercream
1/2cup1 stick unsalted butter at room temperature
1/2cupunsweetened cocoa powder
1tspgood quality vanilla extract
4 to 5cupsconfectioners' sugar
1/4cupmilk
One premade Oreo pie crust
Instructions
For The Cake
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.
For the Oreo filling:
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.
For the chocolate buttercream:
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.
To Assemble
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.
Immediately after frosting the cake, working quickly, cover the cake in Oreo crumbs (from the Pie Crust). Pour a small amount on top of the cake near the edge. Use one hand to move the crumbs all over the edge and the other to catch the crumbs and press them into the side of the cake. Repeat to go around the whole cake. When done with the sides, pour the excess crumbs back onto the top of the cake. Make sure you have a smooth layer on the top and sides.