Cream 2 1/2 cups sugar with butter. Stir in light cream and bring to boil in large pan, stirring so sugar does not cling to sides. Reduce heat to low. Caramelize remaining 1/2 cup sugar by cooking and stirring in heavy skillet over medium high heat til sugar melts and turns brown. Add to cream mixture and continue cooking to soft ball stage. Remove from heat, add vanilla and cool. Beat til thick enough to spread on cake. Makes 2 1/2 to 3 cups icing.