Whisk the powdered sugar and cornstarch together in a small bowl.
Line a 9"×9″ baking pan with parchment paper. Grease the paper, including up the sides of the pan, and heavily dust it with the sugar + cornstarch mix. You should use up about 1/2 of the cornstarch mixture for this.
Pour the gelatin over 1/2 cup of cold water in the bowl of a stand mixer. Allow to rest, undisturbed, until the gelatin absorbs much of the water. This should take a few minutes.
Meanwhile, combine the remaining 1/2 cup of water with the sugar, corn syrup and salt in a medium saucepan. Heat over medium-low, stirring constantly, until the sugar is dissolved. Stop stirring, cover the pan, increase the heat to high and bring to a boil. Let it boil for 3-4 minutes before removing the cover.
Allow the syrup to boil until a candy thermometer says it is 240°F, about 3-5 minutes. Do NOT stir the syrup during this time. Immediately remove the syrup from the heat.
With the stand mixer fitted with a whisk attachment and running on low, gradually pour the syrup into the gelatin. Increase the speed to high and beat until the marshmallow is thick, lukewarm, white and beginning to lose its shine. This should take 12-15 minutes. Add cherry juice and a few drops of food coloring in the last minute or so.
Pour the marshmallow into the lined baking pan. Dust with the remaining powdered sugar and cornstarch.
Allow to rest uncovered at room temperature for 3-4 hours, or until firm.