Line 8x8 pan with parchment paper, and cover the bottom and sides with half of the cornstarch mixture.
Put gelatin and first ½ cup water into the bowl of a stand mixer and set aside.
Heat sugar, maple syrup, corn syrup, and the other ½ cup of water covered over medium-high heat for approximately 4 minutes.
Uncover and continue heating until the mixture reaches 240 F.
Remove from heat.
Begin beating the gelatin mixture with the whisk attachment of the mixer, and very slowly pour in the syrup mixture.
Increase the mixer to high speed (do this slowly to avoid splatter!) and beat an additional 13 minutes.
Spread the marshmallow mixture into the prepared pan. Sprinkle remaining cornstarch mixture over top of the marshmallow mixture.
Let set for 4 hours.
Cut into cubes with a greased knife, or use a greased cookie cutter to cut into shapes.
Notes
Put the gelatin and water in the mixing bowl ahead of heating the mixture in the saucepan (or while it's heating while covered) to make sure it sits for at least 5 minutes before you begin mixing.Grease your spatula with non-stick spray before spreading the marshmallow into your prepared pan to keep the mixture from sticking.