For the Crust:
For the Filling:
For the Topping:
Bittersweet chocolate curls to decorate (we used a hefty brick of bittersweet chocolate)
To make the crust, preheat oven to 350°F. Butter 9-inch-diameter springform pan with 3-inch-high sides. Blend wafer cookies in a food processor until finely ground; blend in sugar. Add melted butter and process until well blended. Crumbs will still be slightly dry. Press crumbs evenly onto bottom (not sides) of prepared pan. Bake just until set, about 5 minutes. Cool while preparing filling. Keep the oven on and maintain temperature.
For the filling, melt the chocolate pieces in a glass bowl in the microwave in 30-second bursts, stirring occasionally, until melted and smooth. Cool chocolate until lukewarm but still pourable. Blend cream cheese, sugar, and cocoa powder in processor until smooth. Blend in eggs 1 at a time. Mix in lukewarm chocolate. Pour filling over crust; smooth top. Bake until center is just set and just appears dry, about 1 hour. Cool 5 minutes. Run small, sharp knife around sides of cake to loosen. Chill overnight.
Next, make the ganache topping. Stir cream, 6 ounces chocolate, and sugar in heavy medium saucepan over low heat until smooth. Cool slightly. Carefully, pour over center of cheesecake, spreading to within 1/2 inch of edge and filling any cracks. Chill in the fridge until topping is set, about 1 hour. Do ahead: Can be made 3 days ahead. Cover with foil and keep refrigerated.
Release pan sides. Transfer cheesecake to platter. Top with chocolate curls. Let stand 2 hours at room temperature before serving.
Note: To make the chocolate curls, we softened the brick of Vahlrhona chocolate in the microwave ever-so-slightly and then shaved curls off using a vegetable peeler.