Step 1: Prepare the Brownie Base

1. Preheat oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
2. In a mixing bowl, whisk together melted butter, sugar, eggs, and vanilla extract until smooth.
3. Sift in cocoa powder, flour, salt, and baking powder, then fold everything together until well combined.
4. Pour the batter into the prepared pan and spread evenly.
5. Bake for 15-18 minutes, then remove and let it cool.

Step 2: Make the Cheesecake Filling

1. In a large mixing bowl, beat cream cheese and sugar until smooth.
2. Add eggs, one at a time, mixing on low speed.
3. Stir in vanilla extract and sour cream, mixing just until combined.

Step 3: Assemble and Bake

1. Pour the cheesecake batter over the cooled brownie base, smoothing the top.
2. Bake at 325°F (163°C) for 35-40 minutes, until the center is slightly jiggly but set at the edges.
3. Turn off the oven and let the cheesecake cool inside with the door slightly open for 30 minutes.
4. Transfer to a cooling rack, then refrigerate for at least 4-6 hours (preferably overnight).

Step 4: Add the Toppings

1. Whip heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
2. Drizzle warm hot fudge sauce over the chilled cheesecake.
3. Pipe or dollop whipped cream around the edges.
4. Sprinkle chopped chocolate on top and add optional garnishes like nuts or cherries.