In the bowl of a stand mixer, pour 1/2 cup of ice cold water. Sprinkle the gelatin over the the water.
In a heavy bottomed saucepan, add the remaining ice water, sugar, corn syrup and salt. Bring to a simmer. Simmer over medium heat until the temperature reaches 240°F on a candy thermometer. This will take around 8 minutes.
Remove from the heat. Turn the mixer speed to medium. Carefully pour the hot sugar liquid down the side of the bowl, keeping it between the bowl and the beater.
Continue until all of the liquid is added.
Add the vanilla. Increase the speed of the mixer to high and whip until thickened, around 15 minutes.
Use a sieve to dust a glass baking dish with powdered sugar.
Use a large spoon or spatula spritzed lightly with cooking spray to scoop from the bowl to the dish.
Spritz an off-set spatula lightly with cooking spray, and spread the marshmallow creme into the dish. [The marshmallow creme is very sticky and the cooking spray will help prevent it from sticking to the utensils while you work.] Work quickly.
Depending on how thick you want the marshmallows to be, you can use an 8 x 12 dish or a 9 x 9 dish for thicker marshmallows.
Dust the top generously with powdered sugar, then allow to sit uncovered on the counter for at least 4 hours or overnight to dry.
To cut, turn onto a cutting board covered in powdered sugar. Use a pizza cutter or sharp knife to cut into squares.
*Unflavored gelatin is available on the baking aisle of most grocery stores along with the flavored gelatin products. The package contains 4 envelopes. Only 3 are needed for this recipe.