To begin, preheat the oven to 350°. Next, line a 9"-x-13" dish with parchment paper and coat with cooking spray. Then, prepare the cake according to the instructions on the package and pour it into the prepared dish. Bake until a toothpick inserted into the center of the cake comes out clean, approximately 25 minutes. Finally, let the cake cool completely.
First, using the back of a wooden spoon, make holes in the cake. Then, pour melted peanut butter all over the cake, ensuring to fill the holes. Lastly, set the cake aside.
Make the frosting: Combine the peanut butter and butter in a large bowl with an electric mixer. Add powdered sugar, vanilla sugar and salt and beat until light and fluffy. Whip the cream a little at a time to loosen the ice if necessary.
First, spread the glaze over the entire surface of the cake. Then, top it with chopped Reese's Peanut Butter Cups and Reese's Pieces.