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.