Cook sugar and 1/3 cup water in a heavy saucepan over medium heat, stirring often, about 15 minutes or until reduced to 6 tablespoons.
Add whipping cream and vanilla. (Mixture will be lumpy.) Cook over medium heat, stirring often, 15 minutes or until reduced to 1 cup. Remove from heat. Stir in butter. Cover and chill up to 3 days, if desired. Serve warm.