Preheat the Oven: Preheat your oven to 325°F (160°C). Grease a 9-inch springform pan or line it with parchment paper.
Prepare the Brownie Layer: Follow the instructions on the brownie mix box, then pour the brownie batter into the prepared pan. Bake for about 20 minutes (until it’s firm but not fully cooked).
Prepare the Cheesecake Layer: While the brownies are baking, beat the cream cheese and sugar until smooth. Add the vanilla, eggs, and sour cream (if using), then mix until well-combined and creamy.
Assemble the Cheesecake: Once the brownie layer is done, pour the cheesecake filling over the slightly undercooked brownies. Spread it out evenly.
Bake Again: Return the pan to the oven and bake for another 40-45 minutes, until the cheesecake layer is set but slightly jiggly in the center.
Cool and Chill: Let the cheesecake cool to room temperature, then refrigerate for at least 4 hours or overnight.
Serve: Top with whipped cream, chocolate chips, or fresh fruit. Slice and enjoy!