Grease a 9 x 13 in [23 by 33 cm] baking pan.

In a small bowl, whisk together the confectioners' sugar and cornstarch.

In a small bowl, combine the gelatin and cold water.

In a medium, heavy-bottom saucepan fitted with an instant-read thermometer, combine the granulated sugar, room-temperature water, corn syrup, and salt. Bring to a boil over medium-high heat until the temperature reaches 240F [115C], 4 to 5 minutes. Immediately remove from the heat and whisk in the gelatin. Set aside.

In the bowl of a stand mixer fitted with a whisk, whip the egg whites and cream of tartar on medium-high speed until soft peaks form, 2 to 3 minutes.

With the mixer running on low speed, pour the hot syrup along the sides of the bowl, being careful not to hit the whisk. Increase the speed to medium-high and whisk until the mixture has doubled in volume and is thick and glossy, and the sides of the bowl have cooled, 8 to 10 minutes. Add the vanilla and mix on low speed until combined.

Scrape the marshmallow into the prepared pan and use an offset spatula to smooth the top of the mixture. Sift 2 to 3 tablespoons of the confectioners' sugar over the top. Let sit overnight at room temperature, uncovered, until firm.

Remove the marshmallow from the pan and cut with a knife, scissors, or pizza wheel dusted with some of the confectioners' sugar mixture. Toss the cut marshmallows (a few at a time) into the remaining confectioners' sugar mixture to coat. Shake off any excess in a fine-mesh sieve. Store at room temperature in an airtight container for up to 1 week.