Add ½ cup cold water in the bowl of a stand mixer fitted with a whisk attachment. Sprinkle the gelatin on top of the water and allow to bloom.
In a medium sauce pan, stir together the corn syrup, ½ cup water, sugar, and salt. Turn the heat to high and bring to a boil. Clip a candy thermometer to the side of the pan and boil until the sugar mixture reached 240°F. Immediately remove from heat.
Turn the mixer on low speed and begin streaming the sugar syrup down the side of the mixer bowl, into the bloomed gelatin. Go slow and be careful not to pour too much at a time. Increase the speed to high and beat for 8 to 10 minutes. Add the vanilla and beat until incorporated.
Pour the marshmallow mixture into the prepared pan and allow it to sit uncovered overnight.
The next day, sprinkle your work surface with a layer of the cornstarch mixture. Turn the marshmallows out onto the surface and cut into desired square sizes with a sharp knife coated with powder mixture. Once cut, dust all sides of each marshmallow with the remaining mixture and your marshmallows are done!
Enjoy them in a cup of hot chocolate, toasted, as a s'more, or just plain!