To Make the Brownie Layer:

1. Preheat oven to 350°F.
2. In a large bowl, whisk together brownie mix, oil, water, and egg.
3. Fold in chocolate chips.
4. Spread batter into the bottom of a well-greased 9-inch springform pan.
5. Bake for about 25 minutes, or until a toothpick inserted 2 inches from the side comes out with moist crumbs.
6. Cool in pan on a wire rack for 10 minutes.
7. Meanwhile, prepare cheesecake layer.

To Make the Cheesecake Layer:

1. In a large mixing bowl, beat cream cheese until smooth.
2. Add sugar, and beat until combined.
3. Add eggs, one at a time, beating well after each addition.
4. Beat in vanilla and creamy peanut butter.
5. Fold in chopped Reese's cups.
6. Pour batter evenly over brownie layer.
7. Return to oven for 40-50 minutes, or until the edges are golden brown, and the center is slightly jiggly.
8. Cool in the pan on a wire rack.
9. Run a knife around the edge, and release the sides of the pan to remove cheesecake.

For the Ganache and Topping:

1. In a small microwave-safe bowl, combine chocolate chips, heavy cream, and butter.
2. Microwave on high at 30-second intervals until chocolate is melted and the mixture is smooth.
3. Pour ganache over cooled cheesecake.
4. Sprinkle with chopped peanut butter cups.
5. Allow 10-15 minutes for ganache to set. Cut and serve.