In a small saucepan, combine the sugar, water and coffee. Bring to a boil; cook for 3 minutes, stirring occasionally. Remove from the heat; stir in butter and rum extract. Cool slightly.
Split cake into three horizontal layers. Place bottom layer on a serving plate. Brush with about 1 tablespoon coffee mixture; spread with 1 cup whipped topping. Repeat layers. Brush remaining coffee mixture over cut side of remaining cake layer; place coffee side down over topping. Spread remaining whipped topping over top of torte. Garnish with grated chocolate if desired. Chill until serving.