In medium saucepan, measure in sugar, water, corn syrup, and salt over medium heat. Stir to dissolve. Cook to soft ball stage, 240 degrees F (115 degrees C), without stirring. It should fall from spoon in a thin thread.
Beat egg whites and cream of tartar until stiff peaks form; do this shortly before syrup has reached the soft ball stage. When ready, pour syrup in a thin stream over stiff egg whites, being careful to beat continuously the whole time. Add vanilla and beat until the right spreading consistency is achieved.
