In a large saucepan, combine sugar, corn syrup, salt and water. Heat to between 234 and 240 degrees F (112 to 116 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface. Remove from heat and beat with an electric mixer until stiff peaks form, 10 to 12 minutes. Stir in vanilla. Pour into prepared pan.
Chill in refrigerator 8 hours or overnight. To cut, loosen edges with a knife. Dust surface with confectioners' sugar, and turn out onto a waxed paper lined surface. Dust with confectioners' sugar again and cut with a knife.
