Preheat oven to 325°F/160°. Wrap the bottom and sides of a 9-inch springform pan with one large piece of foil, then repeat with another piece. This will ensure that water from the bath doesn’t seep into the cake.
Make the crust: In a medium bowl combine crumbs and melted butter and mix until crumbs are evenly moistened. Press mixture into the bottom of the pan to form an even layer. Bake crust for 8 minutes. Set the pan on a wire rack and allow the crust to cool completely before adding the filling. Leave oven on.
Make the cheesecake: In a medium heatproof bowl, combine chocolate, heavy cream, and cocoa powder if using. Microwave in 20- to 30-second increments, mixing in between, until chocolate is melted and mixture is smooth. Set aside to cool slightly. In a mixer bowl fitted with the paddle attachment, beat cream cheese on medium-low speed until smooth and lump-free, about 1-2 minutes. Add sugar and beat until blended. Scrape down the sides and bottom of the bowl as needed. Add eggs and beat until batter is smooth. Mix in chocolate mixture. Add vanilla extract and liqueur if using, and beat just until combined. Don’t overbeat. Pour batter over the baked crust.
Make the topping: In a medium heatproof bowl, combine chocolate and heavy cream. Microwave in 20- to 30-second increments, mixing in between, until chocolate is melted and mixture is smooth. Let cool slightly, then pour over the cake.