Preheat your oven according to the instructions on the cake mix box. Grease and flour a 9×13-inch baking pan.
Mix the Devil's food cake according to the package instructions.
Baking
Pour the cake batter into the prepared pan and bake for the time recommended on the box.
Allow the cake to cool in the pan for about 10 minutes.
Assembly
Once the cake is slightly cooled, use the handle of a wooden spoon to poke holes all over the top of the cake.
In a bowl, mix the peanut butter, milk, and powdered sugar until smooth. Pour this mixture over the cake, making sure it goes into the holes.
Spread the chocolate frosting over the cake, covering the top evenly.
If desired, sprinkle crushed Reese’s peanut butter cups over the frosting for an extra treat.
Final Steps
Let the cake chill in the refrigerator for at least an hour before serving to help the flavors meld together.
Notes
Store any leftover cake in an airtight container in the refrigerator for 3-4 days. The flavors will improve after a day in the fridge. For a richer flavor, slightly heat the peanut butter before mixing.