Bake cake: Preheat oven to 350°. Line a 9″-x-13″ pan with parchment paper and grease with cooking spray. Prepare cake batter according to package instructions and pour into prepared pan. Bake until a toothpick inserted into the middle of the cake comes out clean, about 25 minutes. Let cake cool completely.
Using the back of a wooden spoon, poke holes all over cake. Pour melted peanut butter over the entire cake, filling the holes. Set aside.
Make frosting: In a large bowl, using an electric mixer, cream together peanut butter and butter. Add powdered sugar, vanilla, and salt and beat until light and fluffy. Gradually beat in cream to loosen frosting, if needed.
Spread frosting all over the top of the cake. Top with chopped Reese’s Peanut Butter Cups and Reese’s Pieces.
