01 - Preheat oven to 325°F. Wrap the outside of a 9-inch springform pan tightly with aluminum foil to create a waterproof seal, preventing any water bath leakage.
02 - Pulse 28 Oreo cookies in a food processor until fine crumbs form. Transfer to a bowl and combine thoroughly with melted butter. Press the mixture firmly and evenly into the bottom of the prepared springform pan. Bake for 10 minutes, then 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, approximately 2 to 3 minutes.
04 - Add the melted and cooled dark chocolate along with the unsweetened cocoa powder to the cream cheese mixture. Blend until uniformly combined and no streaks remain.
05 - Blend in the sour cream and vanilla extract just until incorporated. Add the eggs one at a time, mixing on low speed after each addition until just combined. Avoid overmixing to prevent air bubbles. Gently fold in the roughly chopped Oreo cookies by hand using a spatula.
06 - 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 allow the cheesecake to cool inside for 1 hour.
07 - Remove the cheesecake from the oven and allow it to cool completely to room temperature. Refrigerate for a minimum of 4 hours, preferably overnight, for the best texture and clean slicing.
08 - Heat the heavy cream in a small saucepan until it just begins to simmer. Remove from heat immediately and pour over the finely chopped dark chocolate in a heatproof bowl. Let sit undisturbed for 2 minutes, then stir gently until the ganache is smooth and glossy.
09 - Pour the ganache over the chilled cheesecake, spreading it evenly across the top. Sprinkle crushed Oreo cookies over the ganache for garnish. Return to the refrigerator for 30 minutes to set before slicing and serving.