Pour the 1/2 cup of water into a mixing bowl and sprinkle the gelatin over it. Allow to sit for 10 minutes.
Mix the water, sugar and agave syrup in a pot. Bring to a boil and allow to reach 240° on a candy thermometer.
Pour syrup over gelatin and mix on medium high for 5 minutes.
Add vanilla (and gel food coloring if you want to dye them a color) and continue to mix on high for an additional 5 minutes.
After grease a spatula and dump marshmallow into a 9X13 pan that has been lined with parchment paper and sprayed with cooking spray. Dust the top with a few tablespoons of the cornstarch mixture and allow to sit overnight
The next day dust all utensils with the cornstarch mixture and cut marshmallows. Then place marshmallows in a bag with a few tablespoons of the cornstarch mixture and shake to coat the marshmallows.