In a large mixing bowl add 1/2 cup water. Sprinkle the gelatin evenly over the water. Add the remaining 2 tablespoons of water to any dry spots of gelatin. Dissolve the gelatin for 10 minutes.
Add the corn syrup, sugar, salt and 1/2 cup of water into a small saucepan. Attach the thermometer to the side of the pan. Cook the mixture over high heat until it reaches 250 degrees F.
Turn the mixer on to slow speed. Slowly pour the corn syrup mixture down the side of the bowl while the mixer is running. Once the entire mixture is poured into the bowl whisk on high speed for 12 minutes.
With one minute remaining add in the vanilla extract. Whisk until fully incorporated.
Meanwhile, combine the powdered sugar and corn starch into a medium sized bowl, stir well. Set aside.
Spray the inside of a 13×9 baking pan with canola oil. With a sifter, then add an even layer of powdered sugar inside of the pan.
Evenly pour the marshmallow mixture into the pan. Smooth out the top of the marshmallows with a lightly oiled spatula.
Spray the top of the marshmallows with canola oil and add another even layer of sifted powdered sugar mixture. Keep the remaining powdered sugar mixture for later use.
Let the marshmallows set for 5-8 hours.