Combine sugar, corn syrup, hot water, and salt in a heavy-bottomed 2-quart saucepan over medium-low heat. Cook and stir until sugar dissolves and mixture boils. Continue cooking without stirring until the temperature reaches 250°F (120°C) on a candy thermometer, or a syrup drop in cold water forms a rigid ball. Wipe crystals from pan sides with a water-dipped pastry brush.

At the same time the sugar syrup is nearing temperature, whip the egg whites in a large bowl until stiff peaks form.

Slowly pour the hot syrup in a thin stream into the beaten egg whites while beating continuously at medium speed. Increase to high speed and continue beating for about 5 minutes until thick. Add vanilla extract and keep beating until stiff and beginning to lose its gloss.