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 comes to a boil. Continue to cook, without stirring, until mixture reaches 250 degrees F (120 degrees C) on a candy thermometer, or until a small amount of syrup dropped into cold water forms a rigid ball. Frequently wipe crystals forming on the sides of the pan, using a pastry brush dipped in water. Remove from heat.

Just as the syrup is reaching temperature, begin whipping egg whites: In a large glass or stainless steel mixing bowl, beat egg whites until stiff peaks form. Pour hot syrup in a thin stream over beaten egg whites, beating constantly with the electric mixer at medium speed. Increase speed to high, and continue beating for about 5 minutes. Add vanilla; continue beating until the mixture becomes stiff and begins to lose its gloss. If it is too stiff, add a few drops of hot water.

Immediately drop by teaspoonfuls onto waxed paper. For a decorative flair, twirl the top with the spoon when dropping. Let stand until set and dry on the outside, at least 8 hours. Store in a single layer in an airtight container at room temperature.