Using a blender or a food processor, finely chop the dark chocolate chips. Place in a small mixing bowl. Add vanilla wafer crumbs, ground hazelnuts, white sugar and melted butter or margarine.
Mix until well combined then press onto the bottom and up the sides of a 20cm springform tin. Bake in a preheated onen for 15 minutes. Cool.
Filling: In a large bowl beat the cream cheese until fluffy. Gradually add white sugar mixing well. Add the eggs and hazelnut liqueur. Mix until well blended. Coarsely cut the dark chocolate chips then add to the cream cheese mixture. Stir. Pour batter into the cooled crust.
Bake at in a preheated 180 degrees C oven for 1 hour. Let cake cool for 1 hour. Remove outer ring from pan. Then let cool completely.
Topping: Melt dark chocolate chips over hot (not boiling) water. Stir until smooth. Dip 13 hazelnuts into the chocolate, covering one half of each nut. Shake off the excess chocolate. Place on a greaseproof paper lined plate. Chill until set.
Combine the remaining melted chocolate with the sour cream. Stir in the hazelnut liqueur. Spread on top of the cooled cheesecake. Garnish with chocolate dipped hazelnuts.