Generously spray the bottom and sides of a 9×13-inch glass or ceramic baking pan with nonstick cooking spray. (I do not suggested a metal pan, as it can affect the taste of the marshmallows.) Sift the confectioners’ sugar and cornstarch together into a bowl and sprinkle some of it very lightly all around the pan, to dust it. Set the remaining sugar/cornstarch mixture aside.

Place the gelatin and 1/2 cup water in the bowl of a stand mixer fitted with a whisk attachment. (Or, if using a handheld mixer, a regular large mixing bowl.) Whisk/beat on low speed for 1 minute to briefly combine. Allow to sit as you cook the sugar syrup—no need to cover.

Place the remaining 1/2 cup water, the corn syrup, granulated sugar, and salt in a 3-quart heavy-duty saucepan over medium heat. Stir with a wooden spoon until the sugar has dissolved. Attach a candy thermometer to the side of the saucepan. Do not let it touch the bottom of the pan. As the sugar syrup begins cooking, brush down the sides of the pan with a water-moistened pastry brush to prevent the sugar from crystallizing.

Without stirring, brushing down the sides of the pan as needed, cook the sugar syrup until it reaches soft ball stage (when it reaches 235–240°F (113–116°C) on a digital candy thermometer). Remove the pan from heat and let sit for 5 minutes.

Turn the mixer on low speed and slowly pour the hot syrup into the gelatin mixture as the mixer runs. The syrup will be extremely hot, so use caution.

Gradually increase the mixer speed until it reaches high. Whip the mixture for 10–15 minutes, or until it is white, thick, and shiny. If using a handheld mixer, this could take longer. During the last minute of whipping, add the vanilla extract.

Pour/spoon the marshmallow into the prepared pan and smooth out the top with a silicone or offset spatula. Allow to sit, uncovered, at room temperature for at least 6 hours, and up to 48 hours.

Once completely firm, you can cut the marshmallows. To do so, cover a large surface or cutting board with parchment paper. Liberally sprinkle the surface as well as the top of the marshmallow with some of the remaining cornstarch/sugar mixture. Using a sharp knife, loosen the edges of the marshmallow from the pan, and then invert the pan onto the prepared surface. Lift a corner of the pan and use your fingers to help peel the marshmallow away from the pan.

Using a sharp knife, cut the marshmallow into 1.5-inch squares, coating the knife blade with some of the leftover cornstarch/sugar mixture as needed. I find running the blade under hot water and wiping it dry helps to make clean cuts as well. Roll the edges of the marshmallows in the cornstarch/sugar mixture, if desired, so they are no longer sticky.

Store leftover marshmallows at room temperature in an airtight container, with parchment paper between layers, in a cool, dry place for up to 2 weeks.