Toast hazelnuts on baking sheet in a 350 degrees F oven 10 minutes, or until golden brown. Reserve 36 for garnish; chop the rest fine. Combine sugar and cocoa in a heavy saucepan; add butter, hazelnut liqueur, and egg. Cook over low heat, stirring constantly, until butter melts and mixture thickens. Remove from heat. Stir in hazelnuts, vanilla wafer crumbs, and coconut. Press mixture firmly into an 8-inch-square pan; cover and chill 20 minutes.
Meanwhile, prepare Hazelnut Buttercream Frosting. Spread frosting over the hazelnut mixture; cover and chill 25 minutes. Cut into 1-inch squares and remove from pan. To garnish, dip reserved hazelnuts into white chocolate and place on each square. Store in refrigerator.