In a bowl, combine the cornstarch and powdered sugar.
Grease a 9x13 inch baking pan. Sprinkle 1/3 of the cornstarch mixture into the pan and coat all sides.
In a stand mixer fitted with the whisk attachment, pour 1/2 cup cold water in the bowl. Sprinkle the gelatin on top, and let stand 5 minutes.
In a saucepan, combine the sugar, corn syrup, 1/2 cup water, and salt. Cover, and bring to a light simmer.
Uncover, and cook until the mixture reaches 240 degrees (6-7 minutes).
Turn the mixer onto low speed and slowly add the hot mixture down the side of the bowl.
Turn the mixture speed up to high, and whip until the mixture in thick and the bowl is lukewarm to the touch (about 13-14 minutes).
Pour into the prepared pan, and dust with more of the cornstarch mixture.
Let sit uncovered at room temperature for at least 3 hours.
Dust a cutting board with some more cornstarch, and turn out the marshmallows.
Cut into squares, or use festive cookie cutters. Dust all sides of each marshmallow in the cornstarch mixture. Store in an airtight container.
