Make Brownie Layer:
Preheat oven to 350°F (175°C). Grease and line a 9-inch springform pan with parchment paper.
Melt butter and stir in sugar. Beat in eggs and vanilla. In a separate bowl, combine cocoa, flour, salt, and baking powder. Stir dry ingredients into wet until smooth.
Pour into pan and bake 20–25 minutes. Let cool completely.

Make Cheesecake Layer:
Reduce oven to 325°F (163°C).
Beat cream cheese until smooth. Add sugar, then mix in sour cream and heavy cream. Beat in eggs one at a time. Stir in vanilla.
Pour over cooled brownie layer.
Place the springform pan into a roasting pan and pour hot water around it (about halfway up sides).
Bake for 60–70 minutes until edges are set and center slightly jiggles.
Cool in oven 1 hour with door ajar, then chill for at least 4 hours or overnight.

Make Hot Fudge Topping:
In a saucepan, combine sugar, cocoa powder, and salt. Add butter and melt over medium heat. Whisk in cream and cook until smooth and glossy.
Remove from heat, stir in vanilla, and let cool slightly.

Assemble & Garnish:
Spread cooled fudge over the chilled cheesecake.
Top with whipped cream, cherries, nuts, and chocolate shavings or sprinkles as desired.

Serve:
Slice and serve chilled for the perfect bite of brownie, cheesecake, and hot fudge in every forkful.