01 - Preheat oven to 325°F. Wrap the outside of a 9-inch springform pan tightly with aluminum foil to create a waterproof seal against water bath leaks.
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 into the bottom and slightly up the sides of the prepared pan. Bake for 10 minutes, then remove and let cool while preparing the filling.
03 - Using an electric mixer on medium speed, beat softened cream cheese and granulated sugar until completely smooth and creamy, scraping down the bowl as needed. Blend in melted dark chocolate and cocoa powder until the mixture is uniformly rich in color.
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 air bubbles. 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 for 45 to 50 minutes, until the edges are set but the center still has a slight jiggle. Turn off the oven, crack the door open, and let the cheesecake rest inside for 1 hour to prevent cracking.
06 - Transfer the cheesecake to a wire rack and cool completely to room temperature. Cover tightly and refrigerate for at least 4 hours, preferably overnight, for the best texture and clean slicing.
07 - Heat heavy cream in a small saucepan over medium heat until it just begins to simmer around the edges. 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 from the center outward until silky smooth.
08 - Pour the ganache over the chilled cheesecake, spreading evenly with an offset spatula. Sprinkle crushed Oreos on top if desired. Return to the refrigerator for 30 minutes to set the ganache before slicing and serving.