In the bowl of an electric mixer, sprinkle gelatin over 1/2 cup cold water. Soak for 10 minutes. Combine sugar, corn syrup, and 1/4 cup water in a small saucepan. Bring to a boil and boil hard 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. Scrape into a 9 x 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.
Let mixture sit for a few hours. Remove from pan, dredge the marshmallow slab with confectioners' sugar and cut into 12 equal pieces with scissors (the best tool for the job) or a chef's knife. Dredge each piece of marshmallow in confectioners' sugar.