Sift the cornstarch and confectioners' sugar together into a medium bowl.
Place a 9-by-13-inch metal baking pan over a clean kitchen towel and lube generously with nonstick spray. Then, thoroughly dust with one-third of the cornstarch-sugar mixture, tapping and angling the pan to ensure complete coverage all the way up the sides. Return any excess mixture to the bowl for later use. Set pan aside.
Pour 1/2 cup of water into your mixer's work bowl and sprinkle the gelatin over the water. Leave to bloom for 5 minutes. If after 5 minutes you still see dry gelatin, drizzle on just enough additional water to moisten, then set aside for another two minutes. Go ahead and install the bowl onto the mixer and attach the whisk.
Meanwhile, combine 1/2 cup water, the granulated sugar, corn syrup, and salt in a 2-quart saucepan. Place over medium-high heat, cover, and cook until the mixture begins to simmer around the edges of the pot, about 4 minutes. (Yes, you'll have to peek occasionally under the lid.) Uncover and continue to cook, without stirring, until the mixture reaches 240 degrees F, approximately 8 more minutes. If you notice sugar crystals forming on the sides of the pot, use a wet pastry brush to wash down the crystals. When the syrup reaches 240 degrees F, immediately remove from the heat.
Turn the mixer to low and SLOWLY and CAREFULLY pour the hot syrup down the side of the bowl into the gelatin mixture. Do not pour it directly onto the whisk. Once you have added all of the syrup, gradually increase the speed to high and continue whipping until the mixture increases several times in volume, thickens, and the bowl feels just warm to the touch, 13 to 14 minutes in my kitchen. Then, with the mixer still running, add the vanilla and continue beating for another 30 seconds to incorporate. Reduce the speed to stir and gradually lift the mixer head to dislodge most of the marshmallow from the whisk. Turn the mixer off.
Quickly transfer your new fluff to the prepared pan, using a spatula lubed with nonstick spray to spread it evenly. The marshmallow will set up quickly, so don't dawdle. Dust the top with enough of the remaining sugar and cornstarch mixture to lightly cover and allow the marshmallow to sit uncovered for at least 3 hours or, better yet, overnight.
Loosen the marshmallow from the sides of the pan using a small spatula or butter knife. Turn the marshmallow out onto a cutting board and dust with a little more of the sugar-cornstarch mixture. Cut into squares using a pizza cutter or sharp knife rubbed with oil. I usually get 24 big marshmallows, but you can also cut smaller cubes for "mini" marshmallows that are perfect for hot cocoa. Once cut, lightly dust all sides of each marshmallow with the additional cornstarch-sugar mixture. Consume right away or store in an airtight container for weeks, if not months.