1. Line a 9x13 pan with foil and grease the foil with butter. Set aside
2. In a large metal bowl sprinkle gelatin over 1/2 cup water and set aside.
3. In a large heavy sauce pan combine sugar, corn syrup, salt and remaining water. Bring to a boil, stirring occassionally. Cook without stirring until you reach soft ball stage or 240°F on your candy therm.
4.Remove from heat gradually adding to gelatin. Beat on high until doubled and thick about 15 minutes.
5.Beat in vanilla. Spread into prepared foil lined pan. Cover and let stand 6 hours or over night.
6. Using foil lift marshmellows out of pan. With a knife or pizza cutter coated with cooking spray cut into 1 inch squares. Store in air tight container