Combine powdered sugar and cornstarch. Coat your pan with vegetable shortening or oil, then sprinkle the powdered sugar mixture, including the sides.
Start by blooming the gelatin by sprinkling two tablespoons of gelatin over the bowl of water. Let it sit for 5-10 minutes, you will know it's ready when it becomes jelly-like (a bit firm to the touch).
While the gelatin is sitting, prepare the sugar mixture. Heat sugar, water, and corn syrup over medium heat until it starts boiling. Let it boil for 4-5 minutes.
If you are using a candy thermometer, you will need to llow the sugar syrup to reach 240°F. In case you are not using a candy thermometer, you can test the sugar syrup by dropping a tiny bit in cod water. If the syrup turns into a sugar ball then it's ready, if it gets dissolved in the water then you need to keep on boiling.
Now that the sugar syrup and the gelatin are ready, add the gelatin to the bowl. With the mixer on low, slowly pour the hot sugar syrup. When all the syrup is added, start to gradually increase the speed until you reach high speed on your mixer.
Whip mixture for 9-10 minutes until stiff peaks form. Now, you need to work fast.
Pour the mixture into the prepared pan, you can shake the pan a bit to have the mixture even. Dust with some of the powdered sugar mixture that is left and let it sit, at room temperature, for at least 2 hours.
When the marshmallow has fully set, remove it from the pan and cut with an oiled knife, consider re-oiling between cuts to prevent any stickiness.
When all the squares are cut, coat them in the remaining powdered sugar mixture and remove excess before serving or storing.