Mix together sugar,corn syrup,water, cornstarch, butter, and salt in a 2qt saucepan.Heat over a medium heat, stirring continuosly until the mixture reaches the hard ball stage (260*). Remove from heat and stir in vanilla. At this time, a few drops of food coloring can be added. Pour into a lightly buttered platter. When taffy is cool enough to handle, pull the taffy until it is satiny, light in color and begins to hold its shape or stiffens.Cut into bite-size pieces and wrap in pieces of wax paper.