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 any leaking during baking.
02 - Pulse 28 Oreo cookies in a food processor until fine crumbs form. Transfer to a bowl and combine thoroughly with the melted butter. 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 the softened cream cheese and granulated sugar together until completely smooth and free of lumps. Blend in the melted, cooled dark chocolate and cocoa powder until the mixture is uniform in color.
04 - Add the sour cream and vanilla extract, blending just until incorporated. Add the eggs one at a time, mixing on low speed after each addition—avoid overmixing to prevent cracks. Gently fold in the roughly chopped Oreo cookies by hand using a spatula.
05 - Pour the filling 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 rest 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, or preferably overnight, until thoroughly chilled and set.
07 - Heat the heavy cream in a small saucepan until it just begins to simmer—do not boil. Remove from heat and pour over the finely 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 into an even layer. Sprinkle crushed Oreo cookies on top if desired. Refrigerate for 30 minutes to set the ganache before slicing and serving.