Prepare a 9×13 inch (23x33cm) tray, by spraying with cooking spray.
Place the sugar, corn syrup, and 1/2 cup of water in a pot to heat over medium heat. Gently stir once to ensure the sugar mixture is wet. Do not stir after the initial mix, to eliminate sugar crystals from forming. Heat the sugar mixture to the ‘soft ball’ stage of candy at 240 to 245 degrees F (115 to 118 degrees C) on a sugar thermometer or instant-read thermometer. This will take 8 to 12 minutes.
While the sugar mixture is heating, in a large heatproof mixer bowl add the gelatin and 1/2 cup water together (no stirring necessary). This blooms the gelatin (initiates the gelatin to absorb water).
When the sugar mixture is at the ‘soft ball’ candy stage, set the mixer bowl with the gelatin at a low speed. Slowly pour the sugar mixture into the mixing bowl. Turn the speed up to high and mix for about 3 minutes until the volume of the marshmallow starts to grow.
Add the vanilla then continue to mix on high speed for another 2 minutes. When the marshmallow fluff has doubled in volume and is glossy and thick, (it will slowly slide off the mixer blades), pour it into the greased tray. Set aside for 4 to 6 hours at room temperature to set.
Mix equal parts of cornstarch and icing sugar, then sift over the top of the marshmallow.
Flip the marshmallow onto a cutting board. With a sharp knife cut into squares using the cornstarch-icing sugar mixture to keep the slices and squares from sticking to each other.
Store in an airtight container until ready to toast over the campfire.