Combine the powdered sugar and cornstarch in a small bowl. Lightly grease a square 8 inch baking pan with vegetable oil.

Sift the sugar and cornstarch mixture and move around to completely coat the bottom and sides of the pan. Return the remaining sugar mixture to a bowl for use later in the recipe.

Place the gelatin into the bowl of a stand mixer along with ½ cup (4 fl oz/120 ml) of the water. This is called sponging. Have the whisk attachment standing by.

In a small saucepan combine the remaining ½ cup (4 fl oz/120 ml) cup water, granulated sugar, corn syrup and salt. Place over medium heat until the sugar has dissolved. Do not let it boil at this stage.

Clip a candy thermometer onto the side of the pan and continue to cook until the mixture reaches 240°F, roughly 8 to 10 minutes, maybe more. You are aiming to reach soft ball stage on the thermometer. Once the mixture reaches this temperature, immediately remove from the heat.

Turn the mixer on low speed and, while running, slowly pour the sugar syrup down the side of the bowl into the gelatin mixture. Once you have added all of the syrup, increase the speed to high. Continue to whip until the mixture becomes very thick and is lukewarm when you touch the bottom of the bowl, approximately 12 to 14 minutes. In the last minute add the vanilla and mix until combined.

Grease a spatula, and even your hands (believe me this is sticky stuff)

Now, work QUICKLY and scrape the marshmallow into the prepared pan, using your spatula for spreading evenly into the pan. Dust the top with enough of the remaining sugar and cornstarch mixture to lightly cover. Reserve the rest for later. Allow the marshmallows to sit uncovered for at least 4 hours and up to overnight. This is a must so they have time to set.

Turn the marshmallows out onto a cutting board and cut into whatever size you want. Once cut, lightly dust all sides of each marshmallow with the remaining mixture, using additional if necessary.

Store in an airtight container for up to 3 weeks. Do not freeze.