Preheat oven to 350°F. Butter 9-inch-diameter springform pan with 3-inch-high sides. Blend cookies in processor until finely ground; blend in sugar. Add melted butter and process until well blended. Press crumbs evenly onto bottom (not sides) of prepared pan. Bake just until set, about 5 minutes. Cool while preparing filling. Maintain oven temperature.
Stir chopped chocolate in metal bowl set over saucepan of simmering water until melted and smooth. Remove bowl from over water; 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 knife around sides of cake to loosen. Chill overnight.
Stir cream, 6 ounces chocolate, and sugar in heavy medium saucepan over low heat until smooth. Cool slightly. Pour over center of cheesecake, spreading to within 1/2 inch of edge and filling any cracks. Chill 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 if desired. Let stand 2 hours at room temperature before serving.