01 - Preheat oven to 325°F. Wrap the outside of a 9-inch springform pan tightly with aluminum foil to create a waterproof barrier, preventing any water bath leaks from seeping in.
02 - Pulse 28 Oreo cookies in a food processor until finely ground. Transfer crumbs to a bowl and pour in melted butter, stirring until the mixture resembles wet sand. Press firmly and evenly into the bottom of the prepared springform pan. Bake for 10 minutes, then remove and let cool while preparing the filling.
03 - Using an electric mixer on medium speed, beat the softened cream cheese and granulated sugar until completely smooth and creamy, scraping down the bowl as needed. Blend in the melted dark chocolate and cocoa powder until uniformly combined and no streaks remain.
04 - Add sour cream and vanilla extract to the batter, mixing just until incorporated. Add eggs one at a time, beating on low speed after each addition until just combined. Avoid overmixing to prevent excess air, which causes cracking during baking. Gently fold in the chopped Oreo cookies by hand using a spatula.
05 - Pour the filling over the cooled crust and smooth the surface with a spatula. Bake on the center rack for 45 to 50 minutes until the edges are set but the center still has a slight jiggle when gently shaken. Turn off the oven, crack the door open, and allow the cheesecake to rest inside for 1 hour as it gradually cools.
06 - Transfer the cheesecake from the oven to a wire rack and cool completely to room temperature. Cover tightly and refrigerate for a minimum of 4 hours, or ideally overnight for the best texture and cleanest slices.
07 - Heat heavy cream in a small saucepan over medium heat until it just begins to simmer around the edges. Immediately remove from heat and pour over the finely chopped dark chocolate in a heatproof bowl. Let stand undisturbed for 2 minutes, then stir slowly from the center outward until a glossy, smooth ganache forms.
08 - Pour the ganache over the well-chilled cheesecake, spreading evenly to the edges with an offset spatula. Sprinkle crushed Oreo cookies on top if desired. Return to the refrigerator for 30 minutes to set the ganache before slicing with a warm, clean knife.