or until wooden pick inserted in center comes out clean. Cool rectangle on wire rack. Cool layers 10 minutes; remove from pans and cool completely on wire rack. Frost rectangle or fill and frost layers with Caramel Frosting. Sprinkle with chopped nuts or ground nutmeg if desired. Store tightly covered. CARAMEL FROSTING Heat brown sugar, margarine and evaporated milk to boiling in 2-quart saucepan over medium heat, stirring constantly. Boil and stir 2 minutes; remove from heat. Cool about 10 minutes or until slightly thickened, stirring occasionally. Gradually stir in powdered sugar. If necessary, stir in 2 to 3 teaspoons water until frosting is smooth and of spreading consistency. 16 SERVINGS; 435 CALORIES PER SERVING..