Line 9 x 9-inch or 8 x 8-inch pan with aluminum foil and lightly oil it using your fingers or non-stick cooking spray. Set aside.
In the bowl of an electric mixer, sprinkle gelatin over 1/2 cup cold water. Soak for about 10 minutes.
Meanwhile, combine sugar and 1/2 cup water in a small saucepan, whisking only until the sugar is dissolved. Bring the mixture to boil and keep boiling it until it reaches 240 F. (116C)
Carefully pour the boiling syrup into soaked gelatin and turn on the mixer, using the whisk attachment, starting on low and moving up to high speed. Beat for between 5-7 minutes, or until fluffy and warm.
Grease a silicone scraper with neutral oil and transfer marshmallow into the prepared pan. Use your spatula to press the marshmallow into the pan evenly. Dust the powder sugar and corn starch mixture on top. Let mixture sit for a few hours, or overnight, until cooled and firmly set.
Sprinkle a cutting surface very generously with dusting mix. Remove marshmallow from pan and lay on top of the sugar. Dust the top generously with this mixture as well.
Use a large, oiled and sharp knife to cut into squares. Separate pieces and toss to coat all surfaces with the starch and sugar.
Store in an airtight container.