Marshmallows are one thing you may not have thought of making from scratch, but once you know how to make homemade marshmallows you won't want to go back to buying storebought! They are deceptively simple and the flavor is out of this world!
In the bowl of a stand mixer fitted with a whisk attachment, sprinkle gelatin over cold water and let it sit for about 10 minutes to bloom.
Meanwhile, in a small saucepan, mix granulated sugar, corn syrup, water, and salt. Bring to a boil over medium high heat and measure with a candy thermometer until it reaches 240 degrees. Do not turn your back on the sugar as it starts getting hot! It boils over very easily if you're not watching!
With mixer on low speed, slowly drizzle in the hot syrup mixture. Gradually increase the speed to medium high and whip the mixture for 11-14 minutes, until mixture is lukewarm, and thick and fluffy like meringue. Add vanilla extract and mix just until incorporated, no more than a minute.
While the marshmallow is whipping, mix together powdered sugar and cornstarch. Prepare a 9x13 or 9x9 inch pan by spraying with nonstick cooking spray and then dusting with half the powdered sugar mixture. Marshmallows are super sticky so make sure the pan is well coated!
Spray a rubber spatula with cooking spray and use it to quickly scrape the marshmallow into the prepared pan. The marshmallow sets pretty quickly so you will need to smooth out the top immediately! Lightly dust the top with more powdered sugar mixture and let sit at room temperature for at least 4 hours, and up to overnight, to set. Save any remaining powdered sugar mixture.
Flip out the marshmallow slab onto a large cutting board and use a large oiled knife or pizza cutter to cut the marshmallows into 1 1/2-inch squares. You can also use an oiled cookie cutter for fun-shaped marshmallows!
Once cut, toss the marshmallows a few at a time in powdered sugar/cornstarch mix to keep them from sticking.
Store in an airtight container at room temperature for up to 3-4 weeks.