Make the crust: Preheat oven to 325°F (160°C). Grease a 9-inch springform pan. Mix cookie crumbs with melted butter and sugar (if using). Press firmly into the bottom of the pan. Bake for 10 minutes. Let cool while preparing the filling.

1. Prepare the filling: In a large bowl, beat cream cheese and sugar until smooth. Add eggs one at a time, mixing on low speed. Add sour cream, cocoa powder, melted chocolate, vanilla, and salt. Mix until smooth, but do not overmix.
2. Bake the cheesecake: Pour filling over the cooled crust. Place springform pan in a larger roasting pan. Fill roasting pan with hot water halfway up the sides (water bath). Bake for 55–65 minutes, or until the edges are set and center jiggles slightly.
3. Turn off oven, crack the door open, and let cheesecake cool inside for 1 hour. Remove, cool to room temperature, then refrigerate at least 4 hours or overnight.
4. Finish and serve: Top with ganache, whipped cream, or berries before serving, if desired.