Prepare the Crust:
Preheat the oven to 350°F (175°C).
Combine the chocolate cookie crumbs and melted butter in a bowl. Press the mixture evenly into the bottom of a 9-inch springform pan. Bake for 8 minutes and let cool.
Make the Cheesecake Filling:
Reduce oven temperature to 325°F (163°C).
In a large bowl, beat the cream cheese until smooth. Add sugar and cocoa powder, mixing until well combined.
Mix in the eggs one at a time, followed by the melted chocolate and vanilla extract.
Pour the filling over the cooled crust and smooth the top.
Bake the Cheesecake:
Place the springform pan in a larger baking dish. Add hot water to the dish, filling it halfway up the sides of the springform pan to create a water bath.
Bake for 50–60 minutes or until the center is set but slightly jiggly. Remove from the oven, cool to room temperature, then refrigerate for at least 6 hours or overnight.
Prepare the Ganache Topping:
Heat the heavy cream in a saucepan over medium heat until it just begins to simmer. Remove from heat and pour over the chopped chocolate. Let sit for 2–3 minutes, then whisk until smooth.
Pour the ganache over the chilled cheesecake, spreading evenly. Return to the refrigerator until the ganache is set.
Serve:
Remove the cheesecake from the springform pan, slice, and serve. Garnish with chocolate shavings or whipped cream, if desired.