For the chocolate cake: Preheat the oven to 350 degrees F. Coat four 6-inch round cake pans with cooking spray; line with parchment paper.
Combine the flour, granulated and brown sugars, cocoa powder, baking soda, baking powder and salt in the bowl of a stand mixer fitted with the paddle attachment. Mix thoroughly.
In another bowl, mix together the sour cream, oil, milk, vanilla, eggs and egg yolks, then pour into the dry ingredients. Mix on medium speed for 5 minutes.
Adjust the mixer speed to low and add the hot water slowly until fully combined.
Pour the batter into the prepared cake pans and bake until the cakes spring back to the touch or a tester inserted in the center comes out clean, 30 minutes.
For the cookies-and-milk mousse filling: Combine the cream, cookies and pudding mix in the bowl of a stand mixer fitted with the whisk attachment, and mix on medium speed until thickened, 1 to 2 minutes. Continue mixing on high speed until peaks form and the mixture has a mousse consistency.
To assemble the cake: Cut the domed top off of each cake round. Place one round on a serving plate. Using an offset spatula, spread an even layer of the filling on top. Repeat with the remaining layers until the cake is fully stacked. Coat the cake with American Buttercream, then apply fondant.
In a stand mixer fitted with the paddle attachment, beat the butter on medium speed until fluffy. Add the powdered sugar, vanilla and water, and mix on low speed until blended. Increase the speed to medium and continue beating until white in color. Scrape down the sides of the bowl with a spatula, then return the mixer to medium speed for 2 minutes.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.
Recipe courtesy of Stephen Lowry, Showboy Bake Shop, Henderson, NV