Spray a 9” x 13” baking pan by spraying with non-stick baking spray, line with a parchment paper sling and spray the parchment paper as well. Pour ¾ C cold water into a small metal bowl and sprinkle the gelatin over top. Stir to combine and let the gelatin hydrate for at least 5 minutes. Meanwhile, heat 1” of water in a small saucepan until simmering, reduce heat to low and place the bowl of gelatin over the pan like a double boiler to melt the gelatin.
Place the sugar into a 4 qt heavy bottom saucepan and add the remaining ½ C of cold water – stir to moisten the sugar, then add the corn syrup and stir to combine. Using a clean pastry brush dipped in water, brush down the insides of the pot so that no sugar crystals remain. Clip candy thermometer to the side of the pot, place over medium heat and bring to a boil.
While the syrup is boiling, place the egg whites, remaining 1 Tbsp of sugar and kosher salt into the bowl of your stand mixer fitted with the whip attachment and beat until stiff, but not dry peaks form. When the syrup reaches 235F watch closely and continue cooking until the temperature reaches 240F – immediately remove from the heat. With the mixer running on low speed slowly drizzle the hot sugar syrup into the egg whites (don’t scrape out the pot at the end). Add the melted gelatin mixture to the egg whites/syrup and then gradually increase the speed to high and beat for 8 minutes. Reduce speed to low and add the vanilla extract, then increase speed back to high and continue to beat for another 3-4 minutes (beating 11-12 mins I all) until mixture cools, and is thick and billowy. Remove the bowl from the mixer and if desired, swirl in some food coloring at this stage – don’t overmix for a pretty swirl. Spray a spatula with baking spray and then use it to scrape the marshmallow into the prepared pan. Let stand until cooled completely (at least 4 hours), then dust with powdered sugar, cover and let rest overnight.
Use an oiled paring knife to help release the marshmallow from the sides of the baking pan. Place a large sheet of parchment onto a large cutting board and lightly dust the surface with powdered sugar. Use the parchment paper to lift the marshmallow out of the pan and invert it onto the board. Remove the parchment paper and dust the top of the marshmallow with powdered sugar. Use an oiled chef’s knife to cut the marshmallow into squares (whatever size you like!), dusting with more powdered sugar as needed to keep them from sticking together and oiling the knife as needed to keep it from sticking. Store in an airtight container or wrap in an airtight cellophane bag for a festive gift.