01 - Preheat oven to 325°F. Wrap the outside of a 9-inch springform pan tightly with aluminum foil to create a waterproof seal and prevent water bath leaks.
02 - Process 28 Oreo cookies in a food processor until fine crumbs form. Transfer to a bowl and mix thoroughly with melted butter until evenly moistened. Press the crumb mixture firmly and evenly into the bottom of the prepared springform pan. Bake for 10 minutes, then remove and set aside to cool slightly.
03 - Using an electric mixer on medium speed, beat softened cream cheese and granulated sugar together until completely smooth and creamy, scraping the bowl as needed. Blend in the melted cooled chocolate and cocoa powder until uniformly combined.
04 - Add sour cream and vanilla extract, blending just until incorporated. Add eggs one at a time, mixing on low speed after each addition just until combined—do not overmix. Gently fold in the chopped Oreos by hand using a spatula.
05 - Pour the filling over the cooled crust and smooth the top evenly with a spatula. Bake for 45 to 50 minutes until the center is just set with a slight jiggle remaining. Turn off the oven, crack the door open, and let the cheesecake cool inside for 1 hour.
06 - Remove the cheesecake from the oven and allow it to cool completely to room temperature. Refrigerate for at least 4 hours, preferably overnight, until thoroughly chilled and set.
07 - Heat heavy cream in a small saucepan until just simmering—do not boil. Remove from heat and pour over the chopped chocolate in a heatproof bowl. Let sit undisturbed for 2 minutes, then stir gently until smooth and glossy.
08 - Pour the ganache over the chilled cheesecake and spread evenly across the surface. Sprinkle crushed Oreos on top if desired. Refrigerate for 30 minutes to set the ganache before slicing and serving.