1. Bring sugar, water, and corn syrup to a boil in a small saucepan, stirring until sugar dissolves. Cook, undisturbed, until syrup registers soft-ball stage (238 degrees) on a candy thermometer.

2. Meanwhile, whisk egg whites with a mixer on low speed until foamy. Add salt, cream of tartar, and vanilla. Increase speed to medium, and whisk until soft peaks form, about 8 minutes.

3. Reduce speed to low, and pour hot syrup down side of bowl in a slow, steady stream. Increase speed to high, and beat until mixture stops steaming, about 3 minutes. Use immediately.