Preheat the oven to 325°F (160°C). Butter a 9-inch springform pan and line the base with parchment.

Combine cookie crumbs with melted butter until the texture is like damp sand. Press firmly into the bottom and slightly up the sides of the pan. Chill for 10 minutes.

Melt the bittersweet chocolate gently in a double boiler or microwave. Allow to cool slightly but remain pourable.

In a wide bowl, beat the cream cheese until smooth. Add sugar and cocoa; beat until glossy. Add eggs one at a time, mixing gently after each. Fold in sour cream, vanilla, salt, and melted chocolate.

If using, fold in the chopped milk chocolate.

Pour the filling into the chilled crust and smooth the surface. Bake in the center of the oven for 50–65 minutes until the edges are set and the center wobbles slightly.

Turn the oven off, prop the door open slightly, and let the cake rest inside for 30 minutes. Remove and cool to room temperature, then refrigerate for at least 6 hours, preferably overnight.

Warm the heavy cream until just below simmering. Pour over the chopped white chocolate; let sit for a minute, then whisk to a glossy ganache. Spread over the chilled cheesecake and refrigerate until the ganache firms, about 30 minutes.

Garnish with chocolate shavings or dusting of cocoa.