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 265ºF 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.