Place the chocolate wafer cookies and chopped hazelnuts in the bowl of a food processor. Pulse them to fine crumbs. Transfer to a large mixing bowl and add the melted butter and salt. Mix together and press into the bottom of a 10-inch springform pan. Bake for 10 minutes. Cool completely on a wire rack, about 30 minutes.
Make the filling: Place the chocolate chips in a microwave safe bowl and heat in the microwave on high at 30 second intervals, stirring between heating, until the chips are melted and stirred smooth. Set aside
Beat the cream cheese in the bowl of an electric mixer at medium speed until smooth. Gradually add the sugar until well blended. Add the eggs, one at a time, beating on low speed until blended after each addition. Add the melted chocolate, cocoa powder and hazelnut liqueur (or vanilla). Beat at low speed until combined. Add the sour cream and stir together with a large rubber spatula. Pour into the prepared crust.
Bake for 1 hour 20 minutes, or until the cheesecake is set at the edges with just a slight wobble in the center. Remove from the oven and let cool completely in the pan. Refrigerate until firm, about 4 hours or overnight.
Run a small knife between the cheesecake and springform pan, and loosen the collar; remove the collar. Just before serving, top the cheesecake with the chocolate-hazelnut spread, the whipped cream and the chocolate shavings.
Cream-filled chocolate sandwich cookies (Oreos) can be used in place of the chocolate wafers, if desired.To make ahead and freeze, chill the baked cheesecake for 4 hours, or until firm. Do not apply toppings. Wrap in double thickness plastic wrap and then place in a gallon freezer bag with the air removed. Thaw in the refrigerator for 24 hours. Apply toppings just before serving.Most chocolate-hazelnut spreads will firm in the refrigerator, making the cheesecake difficult to slice neatly. Apply the spread just before serving, along with the whipped cream and shavings. If there are leftovers, slice the remaining cheesecake into portions before refrigerating.This recipe was adapted from the Southern Living 1972 Party Cookbook.