Grease a 9×13-inch glass or ceramic baking pan with nonstick spray. Sift together the confectioners’ sugar and cornstarch. Lightly dust the pan with the mixture and set the rest aside.

In the bowl of a stand mixer fitted with a whisk, combine the gelatin and 1/2 cup of cold water. Let it sit while preparing the syrup.

In a saucepan, mix the remaining 1/2 cup water, corn syrup, granulated sugar, and salt. Stir until dissolved. Attach a candy thermometer and heat to 235–240°F without stirring. Brush down sides of the pan with water if needed.

Remove syrup from heat and let it sit for 5 minutes. Slowly pour the hot syrup into the gelatin mixture while mixing on low speed.

Increase the speed to high and whip for 10–15 minutes until thick, white, and glossy. Add the vanilla during the last minute of whipping.

Pour the marshmallow mixture into the prepared pan. Smooth the top and let it sit uncovered at room temperature for at least 6 hours or overnight.

Dust a cutting surface with the remaining sugar/cornstarch mixture. Loosen and invert the marshmallow slab. Cut into squares using a sharp knife dusted with sugar mixture or warmed under hot water.

Roll all sides of the cut marshmallows in the sugar/cornstarch mixture to prevent sticking.