In a food processor, pulse 4 peanut butter cups 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.
Preheat 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. To a large bowl, add the cake mix, 11⁄2 cups evaporated milk, 8 tablespoons butter, the vanilla, salt, eggs, and 1⁄3 cup cool water; using an electric mixer, beat 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.
When all 8 cake layers have cooled, make the icing. In a medium pot, whisk together the remaining milk, the sugar, and the cocoa powder. Add the remaining butter, then cook over medium heat, stirring constantly, until butter is melted and icing is shiny, 4–5 minutes. Let cool for 5 minutes. Stir well.
Place one cake layer on a large, flat plate or cake stand. Spread the surface with 1⁄4 cup of icing, then 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. Let sit for 2–3 hours before serving. Store leftovers in an airtight container in the fridge for up to one week.