Combine egg whites, Karo syrup, sugar and salt in top of double boiler. Place over rapidly boiling water and cook, beating constantly with rotary beater until frosting stands in peaks. Remove from heat. Add flavoring, beating in well. Cool slightly.
For filling: Place 1/3 of the frosting in a small bowl and add remaining ingredients; mix well. Spread this mixture between two 8 or 9-inch layers white cake. (Use packaged white cake mix or your favorite recipe.) Spread top and sides of cake with the plain frosting. If desired, garnish top with circle of chopped fruits and nuts.