In a large mixing bowl, add 1/2 cup water and sprinkle on gelatin. Let sit.
In a saucepan add 1/2 cup water, sugar, corn syrup and salt. Heat over medium high heat until candy thermometer reaches 240°F then immediately remove from the heat.
Using a mixer, slowly add the sugar syrup into the gelatin mixture. Then beat on high until it becomes very thick, about 15 minutes. Then mix in the vanilla.
In a bowl, combine icing sugar and cornstarch. Grease a rimmed baking sheet and sprinkle with some of the icing sugar mixture. Reserve rest.
Pour sugar mixture into pan, and spread out evenly with a greased spatula. Dust some of the icing sugar mixture over top to fully coat. Then let sit uncovered for at least 4 hours.
Flip out the marshmallows out onto a cutting board, then cut into squares using a pizza cutter (dusted with the icing sugar mixture). Then sprinkle the remaining icing sugar mixture over all the cut sides of the marshmallows.
Store in an airtight container for up to 3 weeks.