Preheat oven to 350°F (175°C). Grease the springform pan.
In a bowl, mix butter and sugar until creamy.
Add eggs and vanilla. Stir in cocoa powder.
Fold in flour, salt, and baking powder.
Spread brownie batter into prepared pan evenly.
Bake for 20 minutes. Let cool slightly.
Beat cream cheese and sugar until smooth.
Add eggs, one at a time. Blend gently.
Mix in vanilla and sour cream until smooth.
Pour cheesecake layer over cooled brownie base.
Bake at 325°F (163°C) for 40 minutes.
Cool completely, then refrigerate for 4 hours.
For fudge, heat cream and butter until hot.
Add chocolate chips. Stir until smooth.
Mix in vanilla. Let cool slightly before topping.
Pour fudge over chilled cheesecake evenly.
Top with whipped cream, nuts, and cherry.