Before you start, make a foil sling for a 13×9-inch baking pan using two long sheets of aluminum foil—one running lengthwise, one crosswise—with extra hanging over the edges so you can lift the marshmallows out later. Press the foil neatly into the corners, then spray generously with vegetable oil spray. In a small bowl, whisk together the confectioners’ sugar and cornstarch and set aside.

Pour ½ cup of the cold water into the bowl of a stand mixer fitted with the whisk attachment. Sprinkle the gelatin evenly over the water and let it sit while you make the syrup—about 15 minutes—until the gelatin blooms and becomes very firm.

In a medium saucepan, combine the remaining ½ cup water and the corn syrup. Pour the granulated sugar and salt into the center of the pan (try not to let the sugar touch the sides—this helps prevent crystallization). Bring to a boil over medium-high heat and cook, gently swirling the pan (no spoon stirring here), until the sugar is fully dissolved and the mixture reaches 240°F, about 6–8 minutes.

With the mixer on low speed, carefully pour the hot syrup into the bloomed gelatin, avoiding the whisk and sides of the bowl. Gradually increase the speed to high and whip until the mixture is thick, glossy, and holds its shape—about 10–12 minutes. Scrape down the bowl as needed. Add the vanilla and mix just until incorporated, about 15 seconds.

Work quickly here. Scrape the marshmallow mixture into the prepared pan using a greased rubber spatula and smooth the top. Don’t stress about getting every last bit out of the bowl—the marshmallow starts to set fast as it cools, so spreading it promptly is far more important than scraping obsessively. Dust the top with about 2 tablespoons of the confectioners’ sugar mixture, cover, and let sit at room temperature overnight until firm.