Lightly grease a 9-inch square baking dish with nonstick cooking spray or vegetable oil. I also recommend lightly greasing a spatula. (If you don’t have a nonstick pan, lightly dust it with cornstarch after greasing it or fully line with parchment paper or plastic wrap and grease that lightly.)
Pour ½ cup of water into the bowl of a stand mixer fitted with the whisk attachment. Sprinkle the gelatin over the top of the water and set aside to hydrate while making the syrup.
In a saucepan, combine the granulated sugar, corn syrup, salt, and remaining ½ cup of water. Place over medium heat. Stir until the sugar dissolves and the mixture comes to a boil. Place a candy thermometer into the syrup and continue boiling, without stirring, until the mixture reaches 240°F. Immediately remove from the heat.
Turn the mixer with the bloomed gelatin in the bowl on low speed, and slowly pour the hot syrup into the gelatin. Once all of the syrup has been added, gradually increase the speed to high.
Beat the sugar mixture until very thick, 3 to 5 minutes. You test to see if it’s done by stopping the mixer, dipping the whisk attachment into the mixture and pulling it up; if a thick ribbon falls from the whisk and forms on the mixture in the bowl, it’s done.
Quickly beat in the vanilla, and immediately transfer the marshmallow to the prepared pan. (WARNING: It will be sticky!) Use the greased spatula or wet your fingers to spread the marshmallow evenly and smooth the top. Allow it to sit, uncovered at room temperature for at least 4 hours to set.
Whisk together the confectioners’ sugar and cornstarch in a small bowl, then place in a fine mesh sieve or sifter, and generously dust a cutting board with the mixture.
Carefully remove the slab of marshmallow from the pan and place it on the cutting board. Dust the top with the sugar mixture. Cut the marshmallow into squares, dusting the knife between each cut. Dredge the marshmallows with more of the dusting powder so they don't stick together. Marshmallows should be stored in an airtight container.
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Notes
Use a candy thermometer. Candy thermometers are inexpensive, handy tools that remove the guesswork and stress of getting the syrup to the perfect temperature.
If you do not have a candy thermometer: Use a glass or bowl of cold water. Drop a small amount of the hot sugar syrup into the water. If it forms a pliable ball of sugar, the syrup is ready. If it dissolves, the syrup is not ready. If the ball is very hard, the syrup is too hot.
Marshmallows are sticky! I cannot stress this enough. This is a very sticky treat. So be sure to grease the pan well. If the dish is not nonstick, dust it lightly with cornstarch after greasing it. Alternatively, fully line it with parchment paper or plastic wrap and lightly grease that.
Dust the knife between each cut. Powdering the knife with the sugar and cornstarch mixture between each slice will help to prevent sticking.
Use vanilla beans or vanilla bean paste for a speckled look!