1/2 cup + 1/3 cup cold water
1 cup sugar
1/3 cup light corn syrup
4 egg whites
pinch of salt
about 1/2 cup powdered sugar and 1/2 cup cornstarch, sifted together
1. In a small bowl, sprinkle the gelatin over the 1/2 cup of cold water to dissolve and soften.
2. In a saucepan fitted with a candy thermometer, mix the sugar and corn syrup with 1/3 cup of water. Place over medium-to-high heat.
3. In the bowl of an electric mixer, pour in the egg whites and beat on low speed until frothy. Add the pinch of salt.
4. When the syrup reaches between 210 and 220 degrees, increase the speed of the mixer and beat the whites until they are thick and fluffy (do not over beat).
5. When the syrup reaches 245 degrees, while the mixer is whipping, pour the syrup into the whites. Pour so that the syrup does not fall on the whip, otherwise much of the syrup will splatter onto the sides of the bowl, not into the egg whites.
6. Scrape the gelatin and water into the pan that you used for the syrup and swirl it to dissolve (it should be hot enough from the syrup to dissolve it). Pour the liquefied gelatin into the whites as they are whipping. Add the vanilla and continue to whip for 5 minutes.
7. Dust with a sifter a 11x 17 (approximately) baking sheet evenly and completely with cornstarch mixture. Use a spatula to spread the marshmallows in a layer on the pan. Allow to dry for at least 4 hours or overnight, uncovered.