Prepare the Brownie Base: Preheat your oven to 350°F. Line the bottom of a 9-inch springform pan with parchment paper cut into a circle, lightly spray the pan with non-stick spray, and set aside. Mix the boxed brownie mix, vegetable oil, water, and beaten egg in a medium bowl until just combined. Pour the batter into the pan and bake for 35 minutes. Immediately sprinkle chocolate and peanut butter chips on top while hot. Let the brownie cool while preparing the cheesecake layer. Reduce oven temperature to 325°F.

Make the Cheesecake Layer: Using a stand mixer, or a hand-held mixer, cream the softened cream cheese until smooth. Add sugar and vanilla extract, mixing for 1-2 minutes. Lower the speed and add eggs one at a time, mixing gently. Incorporate the peanut butter and heavy cream, mixing until smooth. Fold in quartered mini Reese’s cups carefully.

Bake in Water Bath: Wrap the springform pan in double-layer aluminum foil. Spoon cheesecake batter on top of brownie layer. Place the pan in a large roasting pan, pour hot water 1 inch up the sides, and bake for 1 hour. After baking, crack oven door open and let cheesecake cool for 30 minutes in the oven. Then allow it to cool on the counter for an additional hour before refrigerating overnight.

Add Ganache and Topping: Heat heavy cream with chocolate chips in 45-second microwave intervals, stirring each time until smooth. Pour ganache over chilled cheesecake and smooth gently. Sprinkle remaining quartered Reese’s cups evenly on top.