Smith Island Cake - Chocolate Peanut Butter Cake

From Saveur Magazine

MAKES ONE 8" CAKE

8 large Reese's peanut butter cups, frozen
Nonstick cooking spray
1⁄4 cup flour
1 18 1⁄4-oz. box yellow cake mix, preferably Duncan Hines
2 cups plus 3 tbsp. evaporated milk
16 tbsp. butter, softened
1 tsp. vanilla extract
1⁄2 tsp. salt
4 eggs
6 cups confectioners' sugar
1⁄4 cup unsweetened cocoa

1. 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.
2. Heat oven to 350°. Grease four 8" round cake pans with cooking spray, dust with half the flour, and knock out any excess. Set aside. Put cake mix, 1 1⁄2 cups evaporated milk, half the butter, vanilla, salt, eggs, and 1⁄3 cup water into a large bowl; beat with an electric mixer until light and fluffy, 10–12 minutes. 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.
3. When all 8 cake layers have cooled, make the icing. Combine remaining milk, sugar, and cocoa in a medium pot; stir well, then add remaining butter. Cook over medium heat, stirring constantly, until butter is melted and icing is shiny, 4–5 minutes. Let cool for 5 minutes. Stir well.
4. Spread a cake layer with about 1⁄4 cup of icing; sprinkle with about 1 tbsp. 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. Let sit for 2–3 hours before serving. The cake can be stored for up to a week refrigerated in an airtight container.