This recipe is from Saveur magazine. "The cake has been around for generations" and is a weekend staple in most homes of Smith Island families. "Every Saturday almost every family makes one."
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 oven to 350°F Grease four 8-inch 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.
Bake until cooked through and golden around edges, 12-14 minutes. Set aside to cool lightly, 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.
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.
Spread a cake layer with about 1/4 cup of icing; sprinkle with about 1 tsp powered peanut butter cups. Top with another cake layer and repeat process to make 8 layers total. 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.