In saucepan, mix 1/2 cup sugar, 2 tablespoons corn starch. Stir in 1/2 cup boiling water, 1/2 square chocolate cut in pieces, 1/4 teaspoon salt. Cook over medium heat until thickened. Stir frequently. Remove from heat. Stir in 2 tablespoons butter and teaspoon vanilla. Spread on cake while frosting is hot. Enough to cover tops only.