Line the bottom of a half-sheet pan (12 x 16 inches) with parchment paper. Generously sift half of the powdered sugar onto the paper.
In the top half of a double boiler OFF THE HEAT, sprinkle gelatin over 1/2 cup COOL water. Make sure that all the gelatin becomes moistened. Set aside at room temperature for 10 minutes to bloom.
In a medium sized saucepan over medium heat, bring to a boil the remaining 1/2 cup of water, sugar and corn syrup.
Warm the gelatin mixture over a double boiler to dissolve. Keep warm.
Once the sugar mixture reaches 230 degrees F, begin whipping the egg whites and salt in a stand mixture to the point of a very soft peak. Once the sugar mixture reaches 245 degrees F, lower the mixer speed to the lowest setting and slowly drizzle the syrup into the egg whites. NOTE: You want to pour it between the whip and the side of the bowl. Whip the mixture at HIGH speed for about 2 minutes to cool the mixture slightly. Return to low speed and then add the dissolved gelatin in the same way you added the sugar syrup. Scrape the gelatin bowl to make sure that is all added to the egg white mixture.
Turn the mixer back to high speed and whip until the mixture has a smooth, light and fluffy consistency - about 5-6 minutes. The mixture will be slightly warm. Add the vanilla.
Pour the mixture into the prepared pan and spread out evenly with an offset spatula. Lightly sift the remaining powdered sugar over the top. Set aside to cool completely. The marshmallows will set up completely in a couple of hours.
Cut into squares with a knife or use a cookie cutter to cut them into shapes. The cut edges will be sticky. Toss the cut marshmallows in powdered sugar.
Make Ahead: Marshmallows will last 4-5 days at room temperature in a tightly covered container.Flavor Variations: You can create different flavors by adding that flavor between steps #5 & #6. (For example: 1/2 tsp cinnamon, lemon oil, orange oil, OR 1/4 tsp peppermint extract). Food coloring can also be added, if desired. Presentation Variations: The cut sides are sticky. Roll the sticky edges in sweetened powdered chocolate, colored sugar, cinnamon-sugar, or toasted coconut.