1In the bowl of an electric mixer, sprinkle gelatin over 1/2 cup cold water. Let rest for 10 minutes.
2Combine sugar, corn syrup, and 1/4 cup water in a small saucepan. Bring to a rolling boil and let cook for 1 minute. Pour boiling syrup into gelatin and mix at high speed. Add the salt and beat for 12 minutes. Add vanilla and incorporate into mixture.
3Scrape into a 9-by-9-inch pan lined with oiled plastic wrap and spread evenly. (Note: Lightly oil hands and spatula or bowl scraper.) After pouring marshmallow mixture into the pan, take another piece of plastic wrap and press mixture into the pan.
4Let mixture sit for a few hours. Remove from pan, dredge the marshmallow slab with powdered sugar, and cut into nine 3-inch-square pieces with kitchen scissors or a very sharp knife. Dredge each piece of marshmallow in powdered sugar.