Preheat your oven to 325°F (163°C). Grease a 9-inch springform pan with butter, then wrap it tightly with aluminum foil.

In a mixing bowl, combine chocolate wafer crumbs with melted unsalted butter until evenly coated. Press this mixture into the bottom of the pan. Bake crust for 10 minutes and let it cool.

Melt the bittersweet chocolate over a double boiler until smooth, then let it cool slightly.

Blend the cream cheese in a food processor until smooth. Gradually add sugar and cocoa powder, mixing well.

With the processor running, add the eggs and heavy cream, blending until just combined.

Fold the melted chocolate and vanilla into the cream cheese mixture gently, then pour this over the cooled crust.

Place springform pan in a larger baking pan and add hot water until about 1 inch up the sides.

Bake for 1 hour and 10 minutes until set but slightly jiggles in the center. Cool on a wire rack.

Refrigerate for at least 6 hours or overnight to firm up and deepen flavors.

For ganache, simmer heavy cream in a saucepan, then pour over chopped chocolate. Whisk until smooth. Add butter and cool slightly.

Pour ganache over chilled cheesecake and let it set at room temperature for 1 hour before serving.