For the devil's food cake: Preheat the oven to 325 degrees F. Grease three 8-inch-round cake pans and line the bottoms with parchment paper.
Combine the hot water, chocolate and coffee powder and whisk to dissolve. Set aside to cool for a few minutes.
Sift the granulated sugar, cocoa powder, flour, baking soda, baking powder and salt into a large bowl and whisk to combine. Add the buttermilk and oil to the chocolate mixture. Add the chocolate mixture, eggs and vanilla to the bowl with the dry ingredients. Beat with an electric mixer on low until the dry ingredients are moistened, about 30 seconds. Give the bowl a scrape with a rubber spatula. Increase the mixer speed to medium and beat the batter for 2 minutes.
Divide the batter equally among the 3 prepared pans, about 1 pound of batter per pan.
Bake until a toothpick inserted into the center of the cakes comes out almost clean, 25 to 35 minutes. Cool in the pans for 12 minutes, then turn out onto wire racks to cool completely. Wrap the cake layers in plastic wrap and refrigerate for 1 hour before assembling.
For the peanut butter buttercream: Beat the butter in a stand mixer fitted with the paddle attachment on medium speed until it is glossy and creamy, 5 to 7 minutes. Reduce the mixer speed to low. Working in batches, slowly add the confectioners' sugar, allowing each addition to be incorporated. Once all the confectioners' sugar has been added, scrape the bowl with a rubber spatula and then beat the mixture on medium speed until no lumps remain and the mixture is smooth and thick, 1 to 2 minutes. Add the milk, vanilla and salt and mix on low speed to combine. Give the bowl and the paddle another scrape, then increase the mixer speed to medium and beat for about 5 minutes. Add the peanut butter and beat until combined. If the icing appears to be too thick, thin it by adding more milk, 1 teaspoon at a time.
To assemble the cake: Place one of your refrigerated cake layers on a cake circle or serving dish and spread about 1 cup of peanut butter buttercream evenly across the cake. Place a second layer on top, press down slightly and spread another 1 cup buttercream evenly over the top of the cake. Top with the third layer. Give the cake a crumb coat by placing a large dollop of buttercream on top of the cake and spreading it to the edges of the cake and down the sides to seal in any crumbs. Refrigerate the 3-layer cake for about 20 minutes. Use the remaining buttercream to frost the top and sides of the cake.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.