In the electric mixer bowl fitted with the whisk attachment, add the gelatin and ½ cup cold water. Let sit while you make the syrup.
While the gelatin is sitting, combine the sugar, corn syrup, salt, and the other ½ cup water in a heavy bottom sauce pan. Cook over medium heat until sugar dissolves.
Raise the heat to high and cook until the syrup reaches 240° on a candy thermometer. Do not walk away from the syrup while it is cooking!
Once it reaches 240°, remove from heat.
With the mixer on low, slowly and carefully pour the syrup into the mixer to combine the gelatin and the syrup.
Raise the mixer speed to high and whip until the mixture is very thick (about 7 minutes).
Mix in the vanilla.
Sift powdered sugar and dust it into an 8 x 12 non-metal baking dish.
Pour the marshmallow mixture into the dusted dish.
Sift more powdered sugar on top of the marshmallow and let sit overnight uncovered to set up.
Turn the marshmallow onto a cutting board and cut into squares.
Notes
Try dipping one side of the marshmallow in melted chocolate.They are also great in coffee or hot cocoa.Pro Tip: I use a Kitchenaid mixer, so my mixing times are shorter that other mixer brands or handheld mixers. If you are using a handheld or other brand of mixer, it should take about 15 minutes to reach the right thickness.