Coat a square baking dish with powdered sugar. 

In a large mixing bowl, add ½ cup cold water and sprinkle gelatin on top. Set aside while the gelatin softens.

Place a candy thermometer on the side of a medium saucepan, then add ½ cup water, sugar, corn syrup, and salt.

Bring to a gentle boil over medium heat and cook until the thermometer reads 240°F (softball).

Using a hand mixer on low, slowly drizzle the hot sugar mixture into the large mixing bowl containing the gelatin mixture.

When all of the sugar has been added, turn the mixer to high and beat for 10 minutes or until stiff peaks form.

When the mixture forms stiff peaks and the bowl has cooled to room temperature, add the vanilla and mix.

Transfer the mixture to the prepared baking pan and work quickly to spread the mixture evenly. 

Sprinkle with additional powdered sugar.

Allow the marshmallows to set overnight at room temperature.

To cut, spread a generous amount of powdered sugar onto a large cookie sheet or cutting board.

Loosen the marshmallows from the edge of the pan using a sharp knife, then remove them and place them on the prepared surface. 

Cut into 1-inch squares and toss to coat with additional powdered sugar, if desired.