Preheat oven to 325°F (160°C). Wrap the bottom of a springform pan in foil to prevent leaks.,Mix chocolate cookie crumbs with melted butter. Press into the bottom of the pan and bake for 10 minutes. Let cool.,Melt chocolate and set aside to cool slightly.,Beat cream cheese and sugar until smooth. Add eggs one at a time, mixing well after each.,Stir in sour cream (or heavy cream), vanilla, and melted chocolate until smooth.,Pour filling over the crust. Place springform pan into a large roasting pan and fill with 1–2 inches of hot water (water bath).,Bake for 55–65 minutes, until edges are set and center slightly jiggles.,Turn off oven, crack the door, and let cheesecake cool for 1 hour inside. Remove and cool completely at room temperature.,Refrigerate at least 4 hours or overnight before serving.,Top with ganache, whipped cream, or chocolate curls before slicing if desired.