In the bowl of a stand mixer fitted with the whisk attachment, combine gelatin with 1/2 cup cold water. Set aside while sugar mixture cooks.Combine sugar, corn syrup, salt, and 1/2 cup water in a small sauce pan and heat over medium - stirring occasionally - until the sugar dissolves. Once mixture is no longer gritty, increase heat to high and cook until a thermometer reads 240 degrees. Remove from heat immediately.Turn the mixer on low and slowly pour the sugar syrup into the dissolved gelatin. Gradually increase speed to high, being careful of steam and splattering, and whip until mixture is thick and fluffy - about 12-15 minutes. Decrease speed to low and add the vanilla. Whip until well combined.Spray a 9x13-inch glass baking dish with cooking spray and dust generously with powdered sugar. Turn the marshmallow mixture into the pan and spread into an even layer. Dust with more powdered sugar. Let it sit - uncovered - at least 8 hours, until the top is only slightly tacky.Turn marshmallows onto a work surface dusted with powdered sugar and cut into squares. Dust cut edges with more powdered sugar and store in an airtight container for up to a month.
Vanilla is perfect for a basic marshmallow, but you can substitute whatever flavoring you want here. 1 tablespoon of any extract of your choosing will work.