Preheat oven to 350°F.
In a large bowl, whisk together brownie mix, oil, water, and egg.
Fold in chocolate chips.
Spread batter into the bottom of a well-greased 9-inch springform pan.
Bake for about 25 minutes, or until a toothpick inserted 2 inches from the side comes out with moist crumbs.
Cool in pan on a wire rack for 10 minutes.
Meanwhile, prepare cheesecake layer.
In a large mixing bowl, beat cream cheese until smooth.
Add sugar, and beat until combined.
Add eggs, one at a time, beating well after each addition.
Beat in vanilla and creamy peanut butter.
Fold in chopped Reese's cups.
Pour batter evenly over brownie layer.
Return to oven for 40-50 minutes, or until the edges are golden brown, and the center is slightly jiggly.
Cool in the pan on a wire rack.
Run a knife around the edge, and release the sides of the pan to remove cheesecake.
In a small microwave-safe bowl, combine chocolate chips, heavy cream, and butter.
Microwave on high at 30-second intervals until chocolate is melted and the mixture is smooth.
Pour ganache over cooled cheesecake.
Sprinkle with chopped peanut butter cups.
Allow 10-15 minutes for ganache to set. Cut and serve.