Preheat oven to 350° and spray two 9″ cake pans with cooking spray. In a large bowl using a hand mixer, beat cake mix, chocolate pudding, sour cream, eggs, and water.
Divide batter among prepared pans and bake until a toothpick comes out clean, 33 to 35 minutes. Transfer to a wire rack to cool completely.
Make the frosting: In another large bowl using a hand mixer, beat butter until smooth. Add about half the powdered sugar and beat until smooth. Add peanut butter and about half the milk and beat until smooth. Add remaining powdered sugar and salt and beat until smooth. (Add additional milk as needed, to thin out the frosting.)
Make ganache: In a small saucepan over low heat, heat heavy cream just until it bubbles. Place chocolate chips in a small heatproof bowl and pour over hot heavy cream. Let sit 2 minutes, then whisk until smooth and no clumps remain.
Assemble the cake: Level cake layers with a large serrated knife. Place a dab of frosting on a cardboard cake circle and top with the first cake layer.
Top cake with 1 cup frosting and smooth into an even layer. Top cake with second cake layer, then frost outside of the cake with remaining frosting.
Drizzle ganache down the sides of the cake, then spiral ganache on top of the cake and smooth top. Pipe a line of frosting around the top edge of the cake to use to build the candy ring around.
Top frosting ring with Reese’s and Reese’s Pieces. (Cake can be stored in an airtight container in the fridge up to 3 days.)