In a large bowl sift and whisk together powdered sugar and cornstarch. Set aside.
100 grams powdered sugar, 100 grams corn starch
Prepare Pan
Lightly spray a 8x8 inch square baking pan with non-stick spray. Next, line the pan with parchment paper that hangs over all 4 sides but lays flat across the bottom of pan. Lightly spray parchment paper with non-stick baking spray.
I like to use an 8x8 metal square with no bottom. Cover a baking pan with a sheet of parchment paper. Spray the inside of the square and then parchment paper with a light coating of baking spray. Place the square on the parchment paper.
Gelatin Bloom
In the bowl of a stand mixer, stir together water and gelatin. The gelatin will absorb and "bloom". Meanwhile make sugar syrup.
18 grams gelatin powder 200 bloom, 90 grams cool water
Sugar Syrup Mix
Combine sugar, water, and corn syrup in a medium saucepan on stovetop and heat on low until sugar has dissolved.
Increase the heat a boil and cook for 6–7 minutes, checking the temperature to reach 240℉ (115℃) on a sugar thermometer.
Turn on stand mixer fitted with whisk attachment to high speed. Then very carefully and slowly pour hot sugar syrup into the gelatin mixture.
Continue to whip on high for 10-12 minutes until tripled in size, thick and fluffy.
Make Marshmallows
Transfer the whipped marshmallow into the prepared pan with rubber spatula and level.
Sift an layer of the powdered sugar and cornstarch coating across the top.
Let sit uncovered in a cool dry place for 4 hrs to set or even overnight.
Lift the marshmallow square from the pan by grabbing the overhanging parchment paper. Or if using a metal square with no bottom, simply remove it.
Flip the marshmallow out onto a cutting surface lightly dusted with powdered sugar/cornstarch mixture. Dust a sharp knife and cut into squares. Coat the sides of marshmallows with more sugar mixture.