Spray a pan with cooking spray. You can use an 8 x 8 pan or for slightly thinner marshmallows I used a 7 x 10 size pan
In your mixer bowl, Sprinkle gelatin over cold waterl and let sit for 5 minutes.
Stir together the sugar, corn syrup, water, and salt in a medium saucepan over high heat.
Bring to a boil and allow to boil, stirring occasionally, until the temperature reaches 240 degrees F
Slowly pour the hot mixture into the mixer bowl over the gelatin, keeping the mixer running.
Increase the speed to medium and beat for 6-7 minutes until white and fluffy.
Beat in the vanilla;
Pour mixture into the prepared pan, using an offset spatula to smooth it into the corners.
Allow to set for several hours. Once set, use a knife to loosen the marshmallow from the edges of the pan.
Turn over onto a powdered sugar dusted work surface, cut into squares with a knife or pizza cutter and toss marshmallows into powdered sugar to coat all sides, shake off the excess.