Begin by preparing a dish in which the marshmallow fluff will cool. We used an 8x8" baking dish, or you could use anything with a rimmed edge no larger than 9x13" and no smaller than 8x8". Prepare the dish by lining the inside with plastic wrap and then dusting powdered sugar inside to coat the bottom. You can set it aside for now.
powdered sugar for dusting
Next, combine the first ½ cup of water and the 3 powdered gelatin packets in the bowl of a stand mixer fitted with the whisk attachment. Give the mixture a quick stir and then let it rest to thicken.
½ C water, 3 packets of powdered gelatin (each 0.25 oz)
Next, combine the second ½ cup water and 2 cups sugar and bring to a boil until it the sugar has dissolved and the mixture reaches a temperature of 230℉.
½ C water, 2 C granulated sugar
Remove the sugar-water mixture from the heat, turn on the stand mixer to medium-high speed and gradually pour in the hot sugar water. This can be done relatively quickly.
Continue whisking the mixture for 10 minutes. It should double in volume and become white and glossy.
After 10 minutes, quickly transfer the marshmallow fluff to your prepared dish and spread evenly using a spatula. Cover with another layer of plastic wrap and transfer it to the refrigerator to cool for 1-2 hours.
After the marshmallows have cooled, you can lift them out by pulling up on the plastic and taking a long knife to cut the fluff into cubes.
Enjoy the marshmallows by themselves, melted in hot chocolate, or roasted over the fire. Store in an airtight container in the refrigerator to avoid a crust forming on the outside for up to one week.