1. In heavy 1-quart saucepan over medium heat, heat sugar, water, corn syrup, and salt to boiling, stirring occasionally. Set candy thermometer in place and continue cooking, without stirring, until temperature reaches 260°F. or hard-ball stage (when small amount of mixture dropped into very cold water forms a ball that is hard enough to hold its shape, yet pliable). Remove saucepan from heat.
2. Just before syrup is ready, in small bowl, with mixer at high speed, beat egg whites until soft peaks form.
3. With mixer at medium speed, pour hot syrup in thin stream into egg whites. Add vanilla and beat at high speed until stiff peaks form, about 5 minutes.