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 - Pulse 28 Oreo cookies in a food processor until fine crumbs form. Transfer to a bowl and mix thoroughly with melted butter until the texture resembles wet sand. Press the mixture firmly and evenly into the bottom of the prepared springform pan. Bake for 10 minutes, then remove and let cool slightly.
03 - Using an electric mixer on medium speed, beat the softened cream cheese and granulated sugar together until completely smooth and free of lumps. Blend in the melted dark chocolate and cocoa powder until uniformly combined and no streaks remain.
04 - Add the sour cream and vanilla extract to the cream cheese mixture, blending just until incorporated. Add the eggs one at a time, mixing on low speed after each addition—be careful not to overmix, as excess air causes cracking. Gently fold in the chopped Oreos by hand using a spatula.
05 - Pour the filling over the cooled crust and smooth the surface with a spatula. Bake for 45 to 50 minutes until the edges are set but the center still has a slight jiggle. Turn off the oven, crack the door open, and let the cheesecake rest inside for 1 hour to cool gradually.
06 - Remove the cheesecake from the oven and allow it to cool completely to room temperature. Transfer to the refrigerator and chill for at least 4 hours, or preferably overnight, until fully set.
07 - Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer—do not let it boil. Remove from heat immediately and pour over the chopped dark chocolate in a heatproof bowl. Let sit undisturbed for 2 minutes, then stir gently in small circles until the ganache is glossy and smooth.
08 - Pour the ganache over the chilled cheesecake, spreading it evenly across the surface. Sprinkle crushed Oreos on top for garnish if desired. Refrigerate for 30 minutes to allow the ganache to set before slicing and serving.