Combine the gelatin and 1/2 cup of water in the bowl of an electric mixer fitted with the whisk attachment.

Stir together the remaining water, sugar, corn syrup, and salt in a medium saucepan. Bring to a boil. Cook until it reaches 240 degrees on a candy thermometer.

Turn your mixer onto low speed and slowly drizzle in the hot liquid, combining with the gelatin. Once you’ve emptied the saucepan turn the speed to medium and let the magic happen. You want to continue stirring until it has thickened and turned solid white (around 10 minutes).

While that is mixing, combine the powdered sugar and cornstarch. Spritz a 9x13 pan with nonstick spray and dust with a portion of the powdered sugar mix. 

Pour your marshmallow into your prepared pan. Dust with a small amount of the remaining powdered sugar mix. Let it sit, uncovered, overnight

Turn the marshmallow out onto a cutting board. Slice into 1 inch pieces and toss in the remaining powdered sugar mixture.