This towering cake is layer-upon-layer of peanut butter and chocolate-y goodness!
Vanilla Sponge Cake
In a medium bowl, sift together the flour, baking powder, and salt.
Cut up the butter into 1-inch pieces and place them in the large bowl of an electric mixer, fitted with a paddle attachment or beaters. Beat for 3 minutes on medium- high speed until the butter is light and creamy in color. Stop and scrape the bowl. Cream the butter for an additional 60 seconds. Add the sugar, 1/4 cup at a time, beating 1 minute after each addition. Scrape the sides of the bowl occasionally. Add the eggs one at a time.
Reduce the mixer speed. Stir vanilla into the buttermilk. Add the dry ingredients alternately with the buttermilk. Mix just until incorporated. Scrape the sides of the bowl and mix for 15 seconds longer.
In a large saucepot over medium high heat bring the milk to a simmer then remove from heat. In a large mixing bowl, whisk the yolks, flour, 1/3 cup confectioners sugar and a pinch of salt together. Add the hot milk in a stream, whisking.
Return the mixture to the saucepot and bring to a boil over moderate heat. Whisk mixture for about 3 minutes until very thick. Transfer to a large bowl. Cover surface of custard with a buttered round of wax paper and cool completely, about 45 minutes.
Using electric beater, add vanilla and remaining cup confectioners sugar to custard. Beat at moderate speed until combined well, then increase speed to medium-high and beat in butter, 2 tablespoons at a time, until smooth. Add chocolates and beat until combined well.
Pulse 4 peanut butter cups in a food processor into small chunks; transfer to a bowl. Pulse remaining peanut butter cups into a fine powder; transfer to another bowl. Chill both until ready to use.
Heat the oven to 350°F. Grease four 8″ round cake pans with cooking spray, dust with half the flour, and knock out any excess. Set aside. Divide half the batter between prepared cake pans. Set remaining batter aside.
Using the back of a spoon, spread out batter so that it covers the bottom of each pan, making it slightly thicker around the edges than in the middle. Bake until cooked through and golden around edges, 12–14 minutes. Set aside to let cool slightly, then loosen cake layers with a knife and invert onto cooling racks. Wash and dry cake pans.
Repeat process a second time with cooking spray and remaining flour and batter.
Once all eight layers have baked, place the first layer on a serving plate. Spread a cake layer with about 3 tablespoons of icing; sprinkle with about 1 tablespoon powdered peanut butter cups. Top with another cake layer and repeat process to make 8 layers in all.
Frost outside of cake with remaining icing; sprinkle top with peanut butter cup chunks.