Place the sugar, agave syrup (or corn syrup), and water in a saucepan. Stir until the sugar is moistened and mixed with the water.
If you’re not using agave or corn syrup - place the sugar in the saucepan and pour the water along the side of the saucepan. Make an “X” in the sugar using your finger - wall to wall (this will help the water spread and moisten the sugar).
Heat over medium-high heat until the sugar syrup comes to a boil. Gently swirl the pan to evenly distribute the heat and dissolve the sugar.
Cover the saucepan with a lid and lower the heat to medium. Let the syrup cook for 2 minutes. Do not lift the lid at this point.
After 2 minutes, remove the lid and check to make sure there are no sugar crystals on the sides of the saucepan. If there are crystals, then cover the saucepan for a further minute to allow condensation that collects on the bottom side of the lid to drip and wash down the sugar crystals back into the syrup.
Clip the sugar thermometer to the side of the saucepan, and set the temperature to between 242 - 245°F. Let the sugar syrup cook until it reaches this temperature - do not exceed 245°F.
If you don't have a sugar thermometer, please see recipe notes for directions to check the correct stage of the sugar syrup.
While the sugar syrup is cooking, prepare the gelatin. Place ½ cup of water in the mixer bowl. Add the gelatin and mix to moisten the gelatin. Then set it aside to allow the gelatin to bloom while the sugar syrup is cooking (at least 10 minutes). Place the bowl in your mixer with the whisk attachment. Add the salt into the bloomed gelatin.
When the sugar syrup reaches the right stage, remove it from the heat and let the bubbles subside. While the mixer is running on medium - low speed, pour the sugar syrup into the bloomed gelatin, along the wall of the bowl (this is to prevent the gelatin from getting scorched by the hot sugar syrup). Increase the speed to medium - high.
Whisk on medium - high for 3 to 5 minutes until the marshmallow base is doubled in size and is white, fluffy and smooth. Then mix for a further 3 to 5 minutes on high speed, allowing the marshmallow to volumize more (up to 3 times). Stop mixing if the marshmallow base has already cooled down, as this will create tough marshmallows. Make sure the base is fluffy, white and up to 3 times the initial volume.
While the marshmallow base is whisking, prepare the pan and utensils that you'll be using. Rub an 8 x 8 inch pan with shortening or butter (you can line this pan with parchment paper if you prefer). Brush all the utensils (spoon, spatulas) with the same fat as well.
Add the peppermint extract during the final minute of whisking.
Once the marshmallow base has tripled in size, remove the bowl from the mixer and add a few drops of red gel coloring into the base. Using the oiled silicone spatula to fold the color into the marshmallow base to create red swirls.
Scrape the marshmallow base into the prepared pan using the same spatula or flexible scraper. Spread the marshmallow evenly in the pan.
Sieve mallow dusting powder generously on top, and allow the marshmallow to set for at least 6 hours.
Turn the set marshmallow out onto a work surface dusted with dusting powder. Cut the marshmallow with an oiled knife or a pair of scissors into 25 square pieces (or 16 pieces for larger marshmallows). Toss the marshmallows in the dusting powder and store in an air-tight container.