Lightly spray a 9×9-inch baking pan with cooking spray. Line pan with parchment paper, leaving a 2-inch overhang on all sides; set aside.
In the bowl of a stand mixer fitted with the whisk attachment, sprinkle the 3 packets of gelatin over 1/2 cup cold water; let stand for 10 minutes. If you don’t have a stand mixer you can use a hand mixer and you want the whisk attachment.

In a 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. Watch it carefully as it can boil and overflow!

Remove the mixture from heat and with the mixer on medium/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 and mix until well combined.

Spray a rubber spatula with cooking spray. Spread marshmallow mixture evenly into pan using prepared spatula. Spray a sheet of plastic wrap with cooking spray and place spray side down, on top of marshmallows.