2 cups confectioners’ sugar, plus extra for dusting
1/4 teaspoon unflavored gelatin
3 cups granulated sugar
4 teaspoons pure vanilla extract
1 1/4 cups light corn syrup
Spray a 9-by-13-by-2-inch baking pan lightly with nonstick cooking spray, then coat the pan generously with confectioners’ sugar.
In a large saucepan, stir ¾ cup water, the granulated sugar, corn syrup, and salt over medium heat until the sugar is dissolved. Insert a candy thermometer and, without stirring, bring to a boil, then cook until the mixture reaches 240°F, about 10 minutes
Meanwhile, pour 3/4 cup water into the bowl of a stand mixer fitted with the whisk attachment, sprinkle the gelatin over the top, and stir on low speed so that the gelatin softens.
With the mixer on low speed, gradually pour the hot sugar mixture into the gelatin mixture, then blend, gradually increasing the speed to high, until the mixture is fluffy and stiff, 10 to 12 minutes. Turn speed to low, add vanilla, mix to incorporate.
Pour the mixture into the prepared pan and smooth the top with a rubber spatula that has been sprayed with nonstick spray. Let the marshmallow sit at room temperature for 4 to 6 hours, until firm
Fill a bowl with 2 cups confectioners’ sugar. Invert the pan onto a smooth work surface and unmold the marshmallow. Lightly coat a serrated knife with nonstick spray, then cut the marshmallow into 12 or 24 squares
Pull apart the squares and toss each marshmallow in the confectioners’ sugar to coat all over. Store in an airtight container for up to 1 week.