01 - Preheat oven to 325°F. Wrap the outside of a 9-inch springform pan tightly with aluminum foil to create a waterproof seal and prevent any leaks during baking.
02 - Pulse 28 Oreo cookies in a food processor until finely ground. Transfer crumbs to a bowl and pour in melted butter, mixing until the texture resembles wet sand. Press the mixture firmly and evenly across 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 lump-free, about 2 minutes. Blend in the melted cooled chocolate and cocoa powder, mixing until the batter is uniformly rich in color.
04 - Add sour cream and vanilla extract, blending just until incorporated. Add eggs one at a time, mixing on low speed after each addition. Stop mixing as soon as the last egg is blended in to avoid incorporating excess air. Gently fold in the chopped Oreos by hand using a spatula.
05 - Pour the filling over the cooled crust and smooth the surface 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 cool gradually.
06 - 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, before adding the topping.
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 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 smooth.
08 - Pour the ganache over the chilled cheesecake, spreading it evenly with an offset spatula. Sprinkle crushed Oreos across the top for garnish. Return to the refrigerator for 30 minutes to set the ganache before slicing and serving.