Grease square pan, 8x8x2 inches, with butter. Mix sugar and cornstarch in 2-quart saucepan. Stir in corn syrup, water, butter and salt. Heat to boiling over medium heat, stirring constantly. Cook, without stirring, to 265°F on candy thermometer or until small amount of mixture dropped into cup of very cold water forms a hard ball that holds its shape but is pliable; remove from heat. Stir in vanilla. Pour into pan.