Preheat the oven to 350˚F. Butter a 9-inch diameter spring form pan with at least 2 ½-inch-high sides. Blend the cookies and sugar in a food processor until crumbled and well blended. Add butter and cacao nibs if using and process until blended and the crumbs are moistened. Press crumb mixture evenly onto bottom of the pan only. Bake until the crust is set, about 8 minutes.
For the filling, stir the chopped chocolate in a metal bowl over a pot of simmering water until melted and smooth. Remove the bowl from over the water; set aside to cool the chocolate until lukewarm but still pourable. In a processor, blend the cream cheese, sugar, vanilla and cocoa powder until smooth. Remember to scrape the bowl down on occasion. Blend in eggs one at a time. Mix in the lukewarm chocolate. Pour the filling over the crust and smooth the top with an offset spatula.
Bake the cheesecake at 350° until the center is just set and the top looks dry—about 1 hour. Cool for 5 minutes. Cut around sides of pan with a knife to loosen the cake. Place warm cake in the pan in the refrigerator and chill uncovered overnight.
Assemble the topping. Stir cream, chopped dark chocolate and sugar in a heavy medium saucepan over low heat until the chocolate melts and the topping is smooth and uniform in color. Cool slightly. Meanwhile, melt the white chocolate on short intervals in a microwave-proof dish; transfer white chocolate to a plastic piping bag (or other heat-proof plastic bag with a corner snipped off to create a small opening). Pour the dark chocolate topping over center of cheesecake and spread to within ½ inch of edge, covering any cracks. Quickly pipe a spiral of white chocolate beginning in the center and working out to the edge of the dark chocolate. Using a knife, start at the center point and draw the tip of the blade edge through both chocolates. Now at the outside edge, skip a space and draw another line in the opposite direction towards the center of the spiral. Repeat in the same manner until a spider web effect is achieved. Chill until topping is set, about 1 hour.
To serve, cut around sides of pan again; remove pan sides. Transfer cheesecake to a platter. Let stand at room temperature for 2 hours before serving.
Can be made 3 days ahead. Cover with foil and keep chilled.