Combine 2 tablespoons cornstarch, dash of salt and 1/2 cup sugar in a saucepan. Gradually stir in 1 1/4 cups of pineapple juice and 1 cup frozen grapefruit juice, diluted according to directions on can. Cook, stirring constantly over moderate heat until thickened. Allow to cook for a few minutes. Remove from heat and stir in butter. Makes 2 1/2 cups of sauce. Serve hot over gingerbread or cake.