01 - Preheat the 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 any batter leakage during baking.
02 - Pulse 28 Oreo cookies in a food processor until they form fine, even crumbs. Transfer to a bowl and mix thoroughly with the melted butter until all crumbs are evenly moistened. Press the mixture firmly and evenly into the bottom of the prepared springform pan, creating a compact, uniform layer. Bake for 10 minutes, then remove and let cool slightly while preparing the filling.
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, mixing until uniformly combined and the mixture is a rich, even color throughout.
04 - Add the 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 until just combined—do not overmix, as this can introduce air and cause cracking. Gently fold in the chopped Oreo cookies by hand using a rubber spatula.
05 - Pour the cheesecake batter over the slightly cooled crust, using a spatula to smooth the surface evenly. 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.
06 - Turn off the oven, crack the oven door open about an inch, and let the cheesecake rest inside the cooling oven for 1 hour. This gradual cooling helps prevent cracks from forming on the surface.
07 - Remove the cheesecake from the oven and allow it to cool completely to room temperature. Transfer to the refrigerator and chill for at least 4 hours, preferably overnight, until fully set and firm.
08 - Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer—do not let it boil. 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 the ganache is glossy and completely smooth.
09 - Pour the ganache over the fully chilled cheesecake, spreading it into an even layer across the top. Sprinkle with crushed Oreo cookies for garnish if desired. Return to the refrigerator for 30 minutes to allow the ganache to set before slicing and serving.