Whisk together the Powdered Sugar and Cornstarch in a small bowl. 1/4 cup Powdered Sugar, 1/4 cup Cornstarch
Lightly grease a 9x13 pan with nonstick spray. Sprinkle a little of the powdered sugar mixture into the bottom of the pan and set it aside.
In the bowl of a stand mixer, or another large bowl, combine 1/2 cup of the water and gelatin. Let it sit for a few minutes until the gelatin blooms. 1 cup Cold Water, 0.75 oz Unflavored Gelatin
Add the remaining 1/2 cup of water, Granulated sugar, and corn syrup to a pot. Cook over medium heat, stirring often until the sugar dissolves, and the mixture starts to boil. 1 1/2 cups Granulated Sugar, 1 cup Light Corn Syrup
Once boiling, add a candy thermometer to the side, and let it cook until it reaches 240°Fahrenheit.
Carefully pour the hot syrup into the large bowl while mixing on low speed.
Once combined, increase the speed and whip for about 10 to 12 minutes, or until it is thick, white, and glossy.
Add in the vanilla, and salt, and mix until just combined. 2 tsp Vanilla Extract, 1/4 tsp Salt
Scoop the marshmallow mixture into the prepared pan and smooth the top with a greased spatula.
Sprinkle a little more of the powdered sugar mixture over the mixture.
Let the marshmallows sit for at least 6 hours, or overnight.
Dust a cutting board or counter top with the powdered sugar mixture and invert the marshmallows onto it.
Using a sharp knife cut into 1.5 inch pieces.
Toss each side with the powdered sugar mixture to keep the pieces from sticking.