Prepare pan. Lightly spray a 9x9-inch baking pan with cooking spray. Line pan with plastic wrap, leaving a 2-inch overhang on all sides; set aside.
Bloom gelatin. In the bowl of an electric mixer fitted with the whisk attachment, sprinkle gelatin over 1/2 cup cold water; let stand for 10 minutes.
Boil sugar, corn syrup and water. In a medium saucepan, combine sugar, corn syrup, and remaining 1/4 cup water. Place saucepan over medium-high heat and bring to a boil; boil rapidly for 1 minute.
Add to gelatin. Remove from heat, and, with the mixer on high, slowly pour the boiling syrup down the side of the mixer bowl into gelatin mixture. Add salt and continue mixing for 12 minutes.
Add vanilla. Add vanilla and mix until well combined.
Spread into pan. Spray a rubber spatula or your hands with cooking spray. Spread marshmallow mixture evenly into pan using prepared spatula or your hands. Spray a sheet of plastic wrap with cooking spray and place, spray side down, on top of marshmallows. Let stand for 2 hours.
Cut marshmallows into squares. Carefully remove marshmallows from pan. Remove all plastic wrap and discard. Cut marshmallows into 1-inch squares using a knife that you have sprayed with non-stick cooking spray or dip in confectioners sugar.
Coat in sugar/cornstarch. Sift confectioners' sugar and cornstarch in a large bowl. Working in batches, add marshmallows to bowl and toss to coat.