Preheat oven to 350°F (175°C). Grease and line a 9-inch springform pan with parchment paper.
Melt butter, stir in sugar, then beat in eggs and vanilla.
In a separate bowl, mix cocoa, flour, salt, and baking powder.
Combine wet and dry mixtures until smooth. Spread batter in the pan and bake for 20–25 minutes. Let cool completely.
Lower oven to 325°F (163°C). Beat cream cheese until smooth, then add sugar, sour cream, and heavy cream.
Mix in eggs one at a time, followed by vanilla extract. Pour the mixture over the cooled brownie base.
Place the springform pan in a roasting pan filled halfway with hot water. Bake for 60–70 minutes until edges are set and the center jiggles slightly.
Turn off oven and let cheesecake cool inside for 1 hour. Then refrigerate for at least 4 hours or overnight.
In a saucepan, whisk together sugar, cocoa, and salt. Add butter and melt over medium heat. Slowly whisk in heavy cream until smooth. Remove from heat, stir in vanilla, and cool slightly.
Spread the fudge over the chilled cheesecake. Garnish with whipped cream, cherries, nuts, or chocolate shavings. Slice and serve chilled.
