Preheat Oven: Preheat your oven to 350°F (175°C) and grease a 9-inch springform pan.
Mix Wet Ingredients: In a large mixing bowl, whisk together the melted butter, granulated sugar, brown sugar, eggs, and vanilla extract until smooth.
Combine Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture until just combined.
Bake the Brownie Layer: Pour the brownie batter into the prepared pan and smooth it out evenly. Bake for 20-25 minutes or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs. Allow to cool completely.
Prepare the Cheesecake Layer:
Mix Cream Cheese Mixture: In a large bowl, beat the cream cheese and sugar with an electric mixer until smooth and creamy. Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract and sour cream.
Layer Over Brownie Base: Once the brownie layer has cooled, pour the cheesecake mixture on top, spreading it evenly.
Bake the Cheesecake:
Bake: Reduce the oven temperature to 325°F (165°C). Bake the cheesecake for 45-50 minutes, or until the edges are set but the center still has a slight jiggle.
Cool and Chill: Turn off the oven and let the cheesecake cool in the oven for 1 hour with the door slightly ajar. Transfer to the refrigerator and chill for at least 4-5 hours or overnight.
Assemble the Sundae Toppings:
Add Hot Fudge: Pour warm hot fudge sauce over the chilled cheesecake, letting it drip down the sides for a dramatic effect.
Decorate: Pipe or spoon whipped cream around the edges, sprinkle with chopped nuts and sprinkles, and top with maraschino cherries.