1/4 c (40 g) confectioners’ sugar
1/4 c (30 g) cornstarch
3 packages unflavored gelatin
1 c (237 ml) ice cold water, divided
1 1/2 c (298 g) granulated sugar
1 c (237 ml) light corn syrup
1/4 tsp salt
1 tsp vanilla
Place the gelatin in the bowl of a stand mixer and add 1/2 c of the water. Set aside.
In a small saucepan, combine the remaining 1/2 cup water, granulated sugar, corn syrup and salt. Place over medium high heat, cover, and cook for 3 to 4 minutes, until the sugar is dissolved. Refrain from stirring the pot; simply swirl it to distribute the granules. Uncover after the sugar has dissolved, and put a candy thermometer into the pot. Continue to cook until the mixture reaches 240F, approximately 7 to 8 minutes. Once the mixture reaches this temperature, immediately remove from the heat.
Turn the mixer on low speed and, while running, slowly pour the sugar syrup into the gelatin, being careful not to hit the beater or the bowl. Once all of the syrup has been added, increase the speed to high. Continue to whip until the mixture becomes very thick and is lukewarm, approximately 12 to 15 minutes. Add the vanilla during the last minute of whipping.
When ready, pour into the prepared pan, using a lightly oiled spatula to spread. Dust the top with enough of the remaining sugar and cornstarch mixture to lightly cover and reserve the rest for later. Allow the marshmallows to sit, uncovered, for at least 4 hours and up to overnight to set.
Turn the marshmallows onto a cutting board and cut into desired shapes, using a greased knife or pizza wheel. Dust all cut sides with the remaining sugar mixture, and store in a covered container until ready to use.