Step 1
Preheat the oven to 350 degrees F (175 degrees C). Grease and line two 9-inch round cake pans with parchment paper.
Step 2
Beat butter and peanut butter for cake together with an electric mixer at medium speed until blended; gradually add brown sugar beating well. Add eggs one at a time beating after each egg.
Step 3
Combine flour baking powder and salt in a bowl; add to peanut butter mixture alternately with milk beginning and ending with flour mixture. Mix at low speed after each addition until blended. Stir in vanilla extract. Pour batter evenly between the prepared cake pans.
Step 4
Bake in the preheated oven until a toothpick inserted into the center of each cake comes out clean 30 to 35 minutes. Remove from the oven and cool completely on a wire rack.
Step 5
While cakes cool place unsweetened and semisweet chocolate in the top of a double boiler; bring water to a simmer and cook until chocolate melts stirring often 3 to 5 minutes. Remove from heat.
Step 6
Beat 1 cup butter with an electric mixer at medium speed until creamy. Add melted chocolate powdered sugar cocoa powder milk and vanilla extract; beat until spreading consistency.
Step 7
Place peanut butter and 1/2 cup butter in a small bowl. Beat with electric mixer at medium speed until creamy. Add powdered sugar gradually alternately with milk until frosting is thick and spreadable. Beat an additional 3 minutes.
Step 8
Remove 1 cooled cake layer from its pan and frost with chocolate buttercream frosting. Place second cake on top and complete frosting with chocolate buttercream.
Step 9
Frost a thick line on the top and bottom edges of the cake with peanut butter frosting using a frosting pouch fitted with a cake decorating tip. Place 16 dollops of peanut butter frosting around the perimeter of the cake. Place mini peanut butter cups on each dollop. Serve immediately.