Preheat oven to 350 degrees F. Butter the bottom and side of a 10-inch springform pan. Wrap a double layer of foil around the bottom and side of pan to form a watertight seal; set aside. For crust: In a small bowl, stir together crushed vanilla wafers and hazelnuts. Stir in the melted butter. Press mixture onto the bottom of the foil-wrapped pan. Set aside.
Prepare Hazelnut Butter; set aside. In a large saucepan, combine bittersweet chocolate and milk chocolate. Cook and stir over low heat until melted. Remove from heat. Stir in the nut butter and chocolate-hazelnut spread.
In a very large bowl, beat eggs with an electric mixer on medium speed until frothy. Gradually add sugar, beating on medium-high speed until mixture is thick, pale, and holds a slowly dissolved ribbon when beaters are lifted. (This will take about 5 minutes in a heavy-duty mixer and about 9 minutes with a handheld mixer.) Pour chocolate mixture into egg mixture; stir gently to combine.
In a medium bowl, beat whipping cream on medium-high speed until soft peaks form (tips curl). Gently fold whipped cream into chocolate mixture. Pour over crust, spreading evenly.
Place the foil-wrapped springform pan in a roasting pan. Pour enough hot water into roasting pan to reach halfway up side of springform pan. Bake for 1-1/4 hours. Turn off oven; let cake stand in oven with door closed for 45 minutes.
Remove springform pan from water. Cool in pan on a wire rack for 2 hours. Using a small sharp knife, loosen edge of cake from side of pan. Remove foil. Cover and chill for 6 to 24 hours.
Let stand at room temperature for 30 minutes before serving. Remove side of springform pan. If desired, sprinkle cake with cocoa powder and serve with whipped cream. Makes 12 servings.
Place toasted hazelnuts (filberts) in a food processor. Cover and process until a paste forms. Add cooking oil. Cover and process with on/off pulses until combined.