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 water bath leaks during baking.
02 - Pulse 28 Oreo cookies in a food processor until fine, uniform crumbs form. Transfer to a bowl and mix thoroughly with 5 tablespoons melted unsalted butter until the mixture resembles wet sand. Press 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. Blend in the melted, cooled dark chocolate and unsweetened cocoa powder until uniformly incorporated and the mixture is silky.
04 - Add sour cream and vanilla extract to the batter, blending just until incorporated. Add the eggs one at a time, mixing on low speed after each addition — avoid overmixing to prevent excess air incorporation. Gently fold in the 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 at 325°F for 45–50 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 to prevent cracking.
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 fully set and firm.
07 - Heat heavy cream in a small saucepan over medium heat until it just begins to simmer — do not boil. Remove from heat and pour immediately over the finely chopped dark chocolate in a heatproof bowl. Let sit for 2 minutes, then stir gently from the center outward until a smooth, glossy ganache forms.
08 - Pour the ganache evenly over the chilled cheesecake, spreading to the edges with a spatula. Sprinkle crushed Oreo cookies on top if desired. Refrigerate for 30 minutes to set the ganache before slicing and serving.