Grease 3 (8") round cakepans; line bottoms with wax paper. Grease and flour wax paper and sides of pans; set aside.
Beat butter at medium speed with an electric mixer until creamy; gradually add brown sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition.
Combine water and vinegar. Combine flour, baking soda, and salt; add to butter mixture alternately with water mixture, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in chopped hazelnuts and vanilla. Pour one-third of batter into each of 2 prepared pans. Fold cocoa into remaining batter; pour chocolate batter into third pan.
Bake at 350° for 19 to 21 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans. Peel off wax paper immediately after inverting. Cool completely on wire racks.
Brush liqueur over cake layers. Place 1 white cake layer, top side down, on serving plate; spread one-third of Chocolate Topping over top (do not frost sides). Top with chocolate cake layer; spread one-third Chocolate Topping over chocolate layer (do not frost sides). Top with remaining white layer and remaining Chocolate Topping (do not frost sides). Garnish, if desired. Allow cake to set several hours before slicing.