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 leaking during baking.
02 - Pulse 28 Oreo cookies in a food processor until they become fine, uniform crumbs. Transfer to a bowl and pour in the melted butter, stirring until all crumbs are evenly moistened. Press the mixture firmly and evenly across the bottom of the prepared springform pan. Bake for 10 minutes, then remove and let 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 cocoa powder, mixing until the batter achieves a uniform deep chocolate color.
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. Stop mixing as soon as each egg disappears into the batter—overmixing will introduce air and cause cracks.
05 - Gently fold the chopped Oreo pieces into the batter by hand using a spatula. Pour the filling over the cooled crust and smooth the surface evenly with a spatula.
06 - Bake 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 the cooling oven for 1 hour. This gradual cooling prevents the top from cracking.
07 - Remove the cheesecake from the oven and let it come to room temperature completely. Transfer to the refrigerator and chill for at least 4 hours, or preferably overnight for the best texture.
08 - Heat the 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 it sit undisturbed for 2 minutes, then stir gently starting from the center outward until the ganache is glossy and smooth.
09 - Pour the ganache over the chilled cheesecake, spreading it into an even layer with an offset spatula. Sprinkle the crushed Oreo garnish over the top. Return to the refrigerator for 30 minutes to set the ganache before slicing and serving.