Whisk powdered sugar and cornstarch together in a medium bow. Line a 9x9 baking pan with parchment and evenly dust with about a quarter of the powdered sugar mixture.
Pour ½ cup water into the bowl of a stand mixer and add gelatin. Whisk together to make sure the gelatin is evenly coated in the water. Place the bowl onto the stand mixer and attach the whisk attachment so it’s ready to go!
Combine the sugar, remaining ½ cup water, and corn syrup in a medium pot. Stir well. If any sugar granules are on the sides of the pot, wipe off with a wet pastry brush.
Cook over medium-high heat until the sugar syrup reaches 240 degrees, about 12 minutes. Turn the stand mixer to low speed and gradually add the sugar syrup. Mix until the gelatin has dissolved, which will take about 1 minute.
Gradually increase the speed to high and mix until the marshmallow is light and fluffy and the mixing bowl is no longer hot to the touch, 10 to 12 minutes.
Now, if you want to flavor your marshmallows, mix in the flavorings (I like ½ tsp rose water or vanilla extract) and then pour the marshmallow into your prepped baking dish. Spray an offset spatula with a little nonstick spray and use it to smooth out the top of the marshmallow.
If it’s dry where you live, loosely cover the pan with a piece of foil, which will keep the marshmallow from losing too much moisture. Allow it to set for at least 12 hours.
Once the marshmallow is set, gently remove it from the pan and the parchment paper. You may need to run an offset spatula between the marshmallow and the sides of the parchment to remove it.
Spray your knife with a little nonstick spray and cut the marshmallow into squares, cut into fun shapes, or leave whole…whatever your heart desires.
Once cut, coat the cut sides of the marshmallows into the remaining powdered sugar mixture and brush off any excess with a dry pastry brush. Use to top fancy hot chocolate or save in an airtight container for late night snacking!