1. In a small bowl, sprinkle the gelatin over the 1/2 cup (125ml) of cold water to dissolve and soften. If using leaf gelatin, soak the leaves in about 2 cups (500ml) cold water.
2. In a small saucepan fitted with a candy thermometer, mix the sugar and corn syrup with 1/3 cup (80ml) of water. Place over medium-to-high heat.
(Note that you will use this saucepan twice, to make the syrup and melt the gelatin, eliminating the need to wash it between uses).
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 about 210ºF (99ºC), increase the speed of the mixer to high and beat the whites until they are thick and fluffy.
5. When the syrup reaches 245ºF (118ºC), while the mixer is running on high speed, slowly pour the hot syrup into the whites, pouring so that the syrup does not fall on the whisk since some of the syrup will splatter and stick to the sides of the bowl.
6. Scrape the gelatin and water into the pan that you used for the syrup, or put the gelatin sheets and 2 tablespoons of the water into the pan and swirl it to dissolve. (There should still be residual heat left in the pan from making the syrup in it to dissolve it).
Pour the liquified gelatin slowly into the whites as they are whipping. Add the vanilla extract or paste and continue to whip for 5 minutes, until the mixture is feels completely cool when you touch the outside of the bowl.
7. Dust a baking sheet evenly and completely with a generous layer of the marshmallow mixture. (I use a sifter to do this.) Make sure there are absolutely no bare spots.
8. Use a spatula to spread the marshmallows in a layer on the pan. Allow to dry for at least 4 hours, preferably overnight, uncovered.
9. Put about 1 cup (140g) of the marshmallow mixture into a large bowl.
Dust the top of the marshmallows with some of the marshmallow mixture. Use a pizza cutter or scissors (dusted as well with the marshmallow mixture) to cut the marshmallows into any size or shape pieces that you’d like and toss the marshmallows in the marshmallow mixture. Shake the marshmallows vigorously in a wire strainer to remove the excess powder.
Alternatively, you can dust a baking sheet and put scoops of the marshmallow on it, and let them cool, as shown in the post.