Pour the gelatin packets into the cold water in a bowl mounted to a stand mixer. Let the gelatin soften and expand for about 20 minutes. Add the optional vanilla flavoring.
While the gelatin is softening, combine the sugar, ¾ cup of water, and corn syrup in a pan over high heat. Stir to dissolve the sugar, then bring the temperature up to 240°F.
Turn the mixer on and slowly pour the hot liquid into the softened gelatin. Once all the liquid has been added, turn the mixer up to medium-high speed. Season with a pinch of salt.
After about five minutes, the gelatin and sugar mixture will turn white. Continue to whip. At around the 10-minute mark, the mix will start forming peaks and begin to increase in volume. Whip for another 10 minutes or until the mixture is light, sticky, and super fluffy.
Line a pan or baking sheet with parchment paper (or use cooking spray). Whisk together the powdered sugar and cornstarch and dust the lined pan.
Pour the marshmallow fluff onto the sheet in an even layer. Dust the top with the remaining powdered sugar mixture.
Let the marshmallows set for about 4 to 5 hours. Cut them into the desired shape and size and serve alongside chocolate squares and graham crackers for DIY s'mores. Enjoy!