Prepare the brownie base: Preheat the oven to 350°F (175°C). Grease and line a 9-inch springform pan. In a bowl, mix melted butter, sugar, eggs, and vanilla. In another bowl, combine flour, cocoa powder, salt, and baking powder. Gradually add the dry ingredients to the wet ingredients. Pour the batter into the pan and bake for 18-20 minutes. Set aside to cool.

1. Prepare the cheesecake layer: Lower the oven temperature to 325°F (165°C). Beat the cream cheese and sugar until smooth. Add the eggs one at a time, then stir in vanilla, sour cream, and heavy cream. Pour over the cooled brownie layer and bake for 55-60 minutes, until the center is slightly jiggly. Let cool in the oven for 1 hour, then refrigerate for 4 hours or overnight.
2. Make the caramel sauce: Melt butter in a saucepan, stir in brown sugar, and bring to a simmer for 3-4 minutes. Add heavy cream, vanilla, and a pinch of salt, then cook for another 1-2 minutes. Remove from heat and let cool to room temperature.
3. Assemble the cheesecake: Once set, drizzle the caramel sauce over the cheesecake or swirl it in for a marbled effect.
4. Serve: Carefully remove the cheesecake from the springform pan, slice, and serve with additional caramel sauce if desired.