1. Prepare the pan by greasing a 9 inch springform pan, lining the bottom with parchment, and wrapping the outside with a double layer of foil to protect it from the water bath.

2. Make the brownie batter by whisking together the melted butter, granulated sugar, eggs, and vanilla, then folding in the cocoa powder, flour, baking powder, and salt just until combined.

3. Spread the brownie batter evenly into the prepared pan and bake at 350°F (180°C) for 18 to 20 minutes, until the edges are set but the center still looks slightly soft.

4. Reduce the oven temperature to 325°F (160°C) and let the brownie base cool slightly while you prepare the cheesecake filling.

5. Beat the softened cream cheese and granulated sugar for the cheesecake until completely smooth, scraping down the bowl often so there are no lumps.

6. Mix in the sour cream, heavy cream for the cheesecake, and vanilla, then sprinkle in the cornstarch and blend just until smooth.

7. Add the eggs one at a time, mixing on low speed just until each egg disappears into the batter to avoid incorporating too much air.

8. Place the foil wrapped springform pan in a large roasting pan, pour the cheesecake batter over the brownie base, and smooth the top.

9. Pour hot water into the roasting pan until it reaches halfway up the sides of the springform pan to create a gentle water bath.

10. Bake the cheesecake for 55 to 65 minutes, until the edges are set and the center still has a slight wobble when you gently shake the pan.

11. Turn off the oven, crack the door, and let the cheesecake cool inside for about 1 hour so it cools slowly and helps prevent cracks.

12. Remove the cheesecake from the water bath, discard the foil, run a thin knife around the edge, and cool completely at room temperature.

13. Cover the cheesecake and refrigerate for at least 4 hours or overnight until fully chilled and firm.

14. Make the hot fudge by heating the heavy cream for fudge until it just begins to simmer, then pouring it over the chocolate chips and butter in a heatproof bowl.

15. Let the mixture sit for a minute, then stir until the chocolate and butter are melted and the mixture is smooth and glossy; stir in vanilla and a pinch of salt for fudge.

16. Allow the hot fudge to cool until thick but still pourable, then spread it over the top of the chilled cheesecake and refrigerate 20 to 30 minutes to set.

17. Whip the cold heavy cream for topping with powdered sugar and vanilla until soft to medium peaks form.

18. Pipe or spoon the whipped cream over the set fudge layer, then sprinkle with chopped toasted nuts and place maraschino cherries on top.

19. Slice with a hot, clean knife, wiping the blade between cuts, and serve chilled for neat pieces and the best texture.