Grease a 9-inch square pan, then line with non-stick baking paper. Make sure to cut excess so that it has flaps on the side. Spray again with cooking spray.
Heat the sugar until it reaches 240F/115C. Once the sugar syrup almost reaches the correct temperature, whip the bloomed gelatin on high speed, slowly and gradually pouring the sugar syrup.
Continue whipping on high speed for about 10-12 minutes until the mixture fluffs.
Dust a chopping board with more cornstarch-powdered sugar, then tip the set marshmallow into the board. Cut using a greased sharp edge knife or greased cookie cutter, as desired. Dust with more powder then sift to remove excess.