Prepare a 9x13 dish with grease and set aside.
In a large bowl or in the bowl of a stand mixer or large bowl, bloom the gelatin. Add the gelatin with ½ cup of water and stir until combined.
In a deep saucepan, combine the remaining water, sugar, corn syrup, and salt. Mix on a medium heat until the sugar is dissolved.
Once the mixture is combined, turn the heat up to medium-high. Let the sugar mixture cook undisturbed until the temperature reaches 240°F. Resist the urge to stir the mixture at any point.
Next, add the sugar mixture to the gelatin. You will want to do this slowly. With the stand mixer on low or with the hand mixture ready to go, add the sugar mixture. Slowly begin beating the mixture. Once the mixture is fully in the bowl, increase the speed. The mixture will look a little pale in color, but that's good. Whip the mixture until it is very fluffy and white in color. This will take anywhere from 7-12 minutes, depending on the speed of your mixer.
Mix in the vanilla and half of the sprinkles.
Once the mixture is super fluffy and cool, spread it into the prepared dish. Once in the pan, top with the remaining sprinkles and cover with powdered sugar.
Place the marshmallow mixture in the refrigerator and allow it to sit for 12 hours or overnight
The marshmallows should spring back when they are ready. Grease a knife or use a cold knife to cut the marshmallows. Roll in powdered sugar to reduce the stickiness of the marshmallow edges.
