Prepare the oreo crust

1. Begin by processing the whole Oreo cookies, including their cream filling, into fine crumbs using a food processor. Alternatively, place the cookies in a sturdy plastic bag and crush them with a rolling pin until no large pieces remain.
2. In a mixing bowl, combine the fine Oreo crumbs with the melted butter and melted chocolate. Stir thoroughly until the mixture has a uniform, wet sand-like consistency and all the crumbs are coated.
3. Transfer this mixture to a springform pan and press it down firmly and evenly across the sides and then press into the bottom to create a compact base.
4. Place the pan in the refrigerator to chill and set while you proceed to the next step.

Prepare Chocolate ganache

1. Chop the dark chocolate for the ganache into small, even pieces and place them in a heatproof bowl.
2. In a small saucepan, heat the heavy cream over medium heat until it begins to simmer gently—do not let it boil.
3. Immediately pour the hot cream over the chopped chocolate and let it sit undisturbed for a minute or two to allow the chocolate to soften. Then, stir the mixture slowly and gently with a spatula until it becomes completely smooth, glossy, and unified.
4. Be careful not to incorporate too many air bubbles. Pour half of this ganache over the chilled crust, spreading it into an even layer. Reserve the remaining ganache for later use. Return the pan to the refrigerator to allow this layer to set.

Make the Chocolate Cheesecake Filling

1. Melt the remaining dark chocolate for the filling using a bain-marie: place a heatproof bowl or pot over a pot of hot water on medium low heat, ensuring the water doesn’t boil and the chocolate doesn’t get too hot. 
2. Stir the chocolate occasionally until it is fully melted and smooth.
3. Remove the bowl from the heat, clean the bottom of the bowl or pot and let the chocolate cool slightly.
4. In a separate large bowl, use a wooden spatula to work the room-temperature cream cheese until it is soft, smooth, and free of lumps. Add the slightly cooled melted chocolate to the cream cheese and mix thoroughly until the two are fully combined and the mixture is uniformly chocolate-colored.
5. In another clean, dry bowl, whip the heavy whipping cream and granulated sugar together until it forms soft peaks—this means the peaks should gently curl over when the whisk is lifted.
6. Gently fold the whipped cream into the chocolate and cream cheese mixture using a spatula, using slow, sweeping motions until the filling is evenly combined and no white streaks remain.

Assemble and Set the Cheesecake

1. Remove the springform pan from the refrigerator/freezer.
2. Pour the prepared chocolate cheesecake filling over the set ganache layer. Use a spatula to spread the filling evenly and smooth the top. Cover the pan with plastic wrap, ensuring it does not touch the surface of the cheesecake. Place the cheesecake in the refrigerator to set for at least 8 hours, though for best results, leave it to chill overnight.
3. This extended time allows the flavors to meld and the texture to become firm enough for slicing.

Decorate and Serve

1. Once the cheesecake is fully set, gently warm the reserved ganache until it is soft and spreadable but not hot.
2. If it becomes too warm, it may split, so heat it in short intervals if necessary.
3. Pour the softened ganache over the chilled cheesecake and use an offset spatula to spread it into a smooth, even layer on top. Return the cheesecake to the refrigerator for an additional 10-15 minutes to allow the ganache to set slightly. When ready to serve, carefully release the springform pan.
4. For clean slices, dip a sharp knife in hot water and wipe it dry between each cut.