Preheat your oven to 350°F. Prepare a 9-inch springform pan by greasing the sides and bottom, then lining the bottom with parchment paper. Wrapping the outside with aluminum foil can help prevent any leaks.

Whisk together the melted butter, granulated sugar, and brown sugar in a large bowl until well combined. Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Sift together the flour, cocoa powder, baking powder, and salt. Gradually fold this dry mixture into the wet ingredients until just combined. The batter should be thick and fudgy, so try not to overmix.

Spread the brownie batter evenly in the prepared springform pan. Use an offset spatula to help create a level base.

Beat the softened cream cheese and sugar with an electric mixer on medium speed until smooth and fluffy, about 2-3 minutes. Add the eggs one at a time, beating well after each addition. Then mix in the vanilla extract and sour cream until just combined.

Carefully pour the cheesecake mixture over the brownie layer, smoothing the top with a spatula.

Bake for 35-40 minutes until the center is almost set but still slightly jiggly.

Allow the cheesecake to cool completely at room temperature, then refrigerate it for at least 4 hours or overnight.

Remove the cheesecake from the springform pan and transfer it to a serving plate.

Drizzle the warm hot fudge sauce over the cheesecake, letting it cascade down the sides.

Pipe or spoon whipped cream around the edges, sprinkle with chopped nuts if desired, and crown each serving with a maraschino cherry and optional sprinkles.