Preheat oven to 325°F (160°C). Lightly butter a 9-inch (23 cm) springform pan and line the bottom with parchment. Wrap the bottom and sides in foil if using a water bath.

Combine cookie crumbs, melted butter, and sugar until the texture holds when pressed. Press evenly into the bottom of the pan and 1/2 inch up the sides. Chill in the fridge while you make the filling.

Gently melt the bittersweet chocolate (double boiler or low-power microwave), stirring until smooth. Keep it warm but not hot.

In a large bowl, whip cream cheese until creamy and smooth. Add sugar and beat until glossy. Add eggs one at a time, mixing gently and incorporating fully without overbeating. Blend in sour cream and vanilla.

Sift cocoa into the batter and fold gently until integrated. Slowly stream the melted bittersweet chocolate into the batter and fold with a spatula until marbled and uniform. Fold in the chopped white chocolate if you want pockets of sweet threads.

Pour the filling onto the chilled crust. Tap the pan gently to release air bubbles and smooth the top with an offset spatula.

For the water bath method: Place the pan in a larger roasting pan. Pour hot water into the outer pan to come halfway up the sides of the springform and bake for 50-65 minutes.

For no-water-bath method: Place the springform on a baking sheet and bake for 45-55 minutes.

Turn the oven off and crack the door, letting the cake cool slowly for 45 minutes. Remove and cool completely on a wire rack, then chill uncovered in the fridge for at least 6 hours or overnight.

Warm the heavy cream until steaming. Pour over chopped semisweet chocolate and let sit for 1 minute. Stir until glossy and smooth, then add a pinch of sea salt. Cool slightly, then pour over chilled cheesecake and smooth.