Preheat oven to 180ºC F. Grease and flour 3 8-inch round cake pans. In a large bowl, measure first 9 ingredients. With mixer at low speed, beat ingredients until just mixed, scraping bowl constantly with spatula. Dissolve 2 tsp instant coffee into 1 cup boiling water; add to ingredients in bowl. Increase mixer speed to medium; beat 2 minutes, scraping sides of bowl occasionally. Pour batter into pans. Bake 25-30 minutes. Cool cake in pans on racks for 10 minutes. Remove from pans and cool completely. While cake is cooling, finely chop Heath Bars. In a cup dissolve half tsp instant coffee in 1 tsp hot water; cool. With serrated knife, cut each cake layer in half to make 6 thin layers. In a large bowl, beat whipping cream, brown sugar and coffee until stiff peaks form. To assemble cake, place 1 layer on cake plate; spread with about half cup of the whipped cream mixture. Reserve about two third of the chopped Heath Bars; sprinkle layer with about one fifth of the remaining Heath Bars. Repeat layering 4 more times. Top with remaining cake layer. Thinly spread remaining whipped cream mixture over side and top of cake. Gently press reserved chopped Heath Bars onto top and sides. Refrigerate cake until ready to serve. Serves 20.