01 - Preheat oven to 325°F (160°C). Wrap the outside of a 9-inch springform pan tightly with aluminum foil to create a waterproof seal and prevent leaks during baking.
02 - Process 28 Oreo cookies in a food processor until fine crumbs form. Transfer to a bowl and mix thoroughly with the melted butter until evenly moistened. Press the 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 the softened cream cheese and granulated sugar together until completely smooth and free of lumps. Blend in the melted cooled chocolate and cocoa powder, mixing until uniformly combined and rich in color.
04 - Add the sour cream and vanilla extract to the mixture, blending just until incorporated. Add the eggs one at a time, mixing on low speed after each addition until just combined—do not overmix. Gently fold in the chopped Oreos by hand using a spatula.
05 - Pour the filling batter over the cooled crust and smooth the top with a rubber 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. Transfer to the refrigerator and chill for at least 4 hours, preferably overnight, until fully set.
07 - Heat the heavy cream in a small saucepan until it just begins to simmer—do not 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 until silky smooth.
08 - Pour the ganache over the chilled cheesecake, spreading it evenly across the surface. Sprinkle crushed Oreo cookies on top if desired. Return to the refrigerator for 30 minutes to set the ganache before slicing and serving.