The best marshmallows ever! They are delicately light and fluffy, perfectly soft and chewy, and they make an unbeatable treat when dipped in dark chocolate or when toasted perfectly golden brown for s’mores.
In a medium mixing bowl whisk together powered sugar and cornstarch. Set aside.
Line a 13 by 9-inch baking dish (for smaller marshmallows) or9 by 9-inch baking dish (if you want larger marshmallows) with two sheets of aluminum foil leaving a 1-inch overhang on all sides. Spray foil with non-stick cooking spray. Spray a spatula with non-stick cooking spray and set near baking dish.
Add 1/2 cup water to bowl of electric stand mixer. While whisking sprinkle gelatin in and whisk quickly for a few seconds, scrape down any gelatin from the sides. Let mixture rest 15 minutes until firm. Securely attach mixer bowl to stand mixer and set stand mixer with a whisk attachment.
Meanwhile pour remaining 1/2 cup water and the corn syrup into a medium saucepan. Pour sugar and salt into the center of the saucepan (so it doesn't touch sides and burn).
Set over medium-high heat and bring to a boil. Let boil, while occasionally gently swirling pan for even cooking, until mixture reaches 240 degrees on a candy thermometer, about 6 to 8 minutes.
Right away set the stand mixer to low speed and carefully pour the hot sugar mixture into the gelatin mixture while avoiding the whisk. Gradually increase speed to high then whip until mixture is very fluffy, well thickened and coats whisk, about 5 to 12 minutes.*
Add the vanilla and continue to blend on high speed until well combined, about 15 seconds longer.
Quickly scrape the mixture into the prepared baking dish then use the greased spatula to spread into an even layer. Sift 2 Tbsp of the powdered sugar mixture evenly over the top. Cover pan and let rest 12 hours for marshmallows to set completely.
Dust a cutting board evenly with 2 Tbsp of the powdered sugar mixture and lightly spray a large chef's knife with non-stick cooking spray.
Invert marshmallows onto cutting board then peel away the foil. Sift 2 Tbsp powdered sugar mixture evenly over top of the marshmallow slab.
Cut marshmallows into rows then to desired individual size (for the smaller marshmallows cut 1-inch squares for larger marshmallows 1 1/2-inch squares).
Working with about 5 marshmallows at a time transfer them to the remaining powdered sugar mixture and toss to coat all sides. Place in a sieve and shake off excess powered sugar back into bowl.
Store at room temperature up to 2 weeks in an airtight container.
Notes
*Recipe originally lists 10 to 12 minutes but I found my mixture to be done around only 5 minutes so this time seems to vary quite a bit based on mixers and so forth. Just pay attention to the fluffiness and thickness of the mixture (also it should be lukewarm at about 95 to 100 degrees).