Butter a large jelly roll pan or cookie sheet with sides really well.
In a medium saucepan, stir together the sugar and cornstarch. Add corn syrup, water, butter and salt and stir well until butter is melted.
Bring to a boil over medium heat and stop stirring. Cook mixture until candy thermometer read 250 degrees F (120 degrees C). This takes some time to get it to 250 degrees. Once it approaches 250 degrees watch very closely because it cooks quickly at the end.
Immediately remove from heat. (If you cook it too long it will turn out very hard.) Carefully stir in vanilla and drink mix. Stir well.
Pour mixture onto buttered baking pan. Allow to cool enough to handle, about 15 minutes. (Try to pull as soon as it’s comfortable to handle.)
Butter hands and begin pulling, as many time as it takes to become opaque and lighten in color, usually about 15 minutes of pulling. You will probably need to butter your hands a few times.
Pull into long rops and cut into bite sized pieces with buttered scissors or a buttered pizza cutter.
Wrap the pieces of taffy in squares of waxed paper and make sure to twist the ends really well to seal.
Store in an airtight container in a cool dry place.