- Preheat oven to 325°F (163°C). Grease a 9-inch springform pan and wrap the bottom in two layers of foil. 
- Crush Oreos into fine crumbs using a food processor. Mix with melted butter and press firmly into the bottom of the pan. 
- Bake the crust for 10 minutes, then set aside to cool. 
- In a large bowl, beat cream cheese until smooth and creamy. Add sugar, cocoa powder, and vanilla, mixing well. 
- Add melted chocolate and sour cream. Beat until combined. 
- Add eggs one at a time, beating on low and scraping down the sides. 
- Pour filling over cooled crust. Tap pan on counter to release air bubbles. 
- Place pan in a larger roasting pan and fill outer pan with hot water halfway up the sides. 
- Bake for 55–65 minutes, until center is almost set. It should still jiggle slightly. 
- Turn off oven, crack the door, and let cheesecake sit inside for 1 hour. 
- Remove from oven, cool to room temp, then chill at least 6 hours or overnight.