Prepare an 8x8 baking pan by lining it with parchment paper, cooking spray, and sprinkling with confectioners sugar. set aside
In the bowl of a stand mixer place gelatin and ½ cup of cold water. give it a quick mix and set aside to bloom while making the sugar mixture
in a medium saucepan add ½ cup of cold water, corn syrup, granulated sugar, and salt.
over medium heat, stir the mixture very slowly until it has all dissolved. turn the heat up to high and bring the temperature of the sugar up to 240°f. leave the mixture alone and do not stir while raising the temperature. too much agitation will increase crystallization.
once it reaches 240°f remove it from heat.
with the whisk attachment on the stand mixer start on a low speed while slowly adding the hot sugar down the side of the bowl to the bloomed gelatin.
once everything is combined slowly up the speed one setting at a time as the mixture gets thicker and thicker till you get to the highest speed. (if you go too fast too soon the very hot sugar will fly everywhere)
keep whipping the mixture until it forms hard peaks. add vanilla and stir until combined.
pour the mixture into the prepared baking sheet, and sprinkle top with an even coating of confectioners sugar. leave overnight to set up.
the next day, turn the marshmallow out onto a surface that has been prepared with a sprinkle of confectioners sugar. using a sharp knife cut into desired sizes. coating the knife with some sugar can help them not stick to the knife.
toss the marshmallows in more confectioners sugar to be sure all sides are covered. shake off excess sugar and you are all done.