Preheat the oven to 350 degrees. Spray the bottom of a 9-inch springform pan with cooking spray, and cut out a round piece of parchment paper to lay on the bottom of the pan. Wrap the springform pan with 2 layers of heavy-duty foil making sure that the bottom of the pan is covered tightly to prevent water from leaking into the crust. (Alternatively, you can place the springform pan in a silicone springform pan protector.)

Mix the Oreo crumbs and butter until the crumbs are coated evenly with butter. Firmly pack the crumbs into the bottom of a 9-inch springform pan. Bake for 10 minutes then set it on a cooling rack to rest while you make the filling. Lower the oven temperature to 325°F.

Place the chopped chocolate in a small, heat-proof bowl. Set aside. Heat the heavy cream in a small saucepan over medium heat until it starts to sizzle around the edge of the pan (or it reaches 180°F). Pour the heavy cream over the chopped chocolate pieces and stir until the chocolate is completely melted and smooth. Set aside and allow to cool.

In a large mixing bowl, beat the softened cream cheese and sugar with an electric mixer on medium speed until smooth and creamy. Mix in the sour cream, dark cocoa powder, and vanilla extract until smooth.

Pour in the cooled chocolate ganache and continue mixing. Add the eggs one at a time, beating at low speed until all of the eggs have been incorporated and they are just mixed. Be careful not to overmix the cheesecake batter once the eggs are added because overmixing can result in a cracked cheesecake.

Pour the cheesecake filling into the baked crust, and place it in a large roasting pan. Fill the roasting pan with hot water that goes halfway up the cheesecake pan. Bake at 325 degrees for 1 hour and 10 minutes or until the cheesecake is still slightly jiggly in the middle.

Turn off the oven but keep the cheesecake in the oven for 1 hour before removing it. Set the cheesecake on a cooling rack for 1 hour or until it is room temperature, then cover it with plastic wrap and chill for at least 4 hours or overnight.

As you did in the first step of making the filling, place the remaining chopped chocolate in a small, heat-proof bowl. Set aside. Heat the heavy cream in a small saucepan over medium heat until it starts to sizzle around the edge of the pan (or it reaches 180°F). Pour the heavy cream over the chopped chocolate pieces and stir until the chocolate is completely melted and smooth. Set aside and allow to cool for 15 minutes.

Use a thin knife to loosen the cheesecake from the pan, then carefully remove the ring of the pan. Carefully transfer the cheesecake to a cake platter or stand, then pour the ganache on top. Spread it out evenly, and let it rest for about 30 minutes.

Garnish with chocolate whipped cream or chocolate shavings, if desired, and serve chilled.