Heat the sugar: Add the sugar and first measure of water to a small saucepan and gently swirl the pot around until the sugar has mostly dissolved trying not to splash too much up on the side of the pot. Clip a candy thermometer to the side of the pot ensuring that it is not sitting on the base of the saucepan (see recipe note 1) Bring to the boil over a medium heat, without stirring and continue to cook until the temperature on the thermometer reaches 115°C/240°F. The temperature will get to 100°C/212°F quickly and will seem like it's taking forever to increase to 115°C, but it will! This whole process will take about 15 minutes. For me, it takes about 8 minutes for the sugar to come to the boil and 7 minutes from then to reach 115°C/240°F. See recipe note 2 for making this without a thermometer.
Prepare the gelatin: While the sugar water is boiling, dissolve the gelatin in the second measure of water in the bowl of a stand mixer or large mixing bowl and allow to sit for 10 minutes.
Whisk together: Once the sugar syrup has reached 115°C/240°F, remove it from the heat and let it sit for a couple of minutes. Using the whisk attachment on a stand mixer and with the mixer running on low/medium speed, slowly pour the sugar syrup into the gelatin mixture. If you don't have a stand mixer, carefully do this while using a hand mixer.Add the vanilla and the salt, increase the speed to high and whisk for 3-5 minutes until doubled in volume and is white, fluffy and smooth.
Prepare the pan: While the marshmallow is whisking, spray a square or rectangle slice/brownie tin with non-stick cooking spray (see recipe note 3) You may line the tin with baking paper first to make it even easier to remove the marshmallow.
Using a lightly oiled rubber spatula, scrape the marshmallow into the baking tin and spread it evenly. Sprinkle the top with marshmallow dust and allow to set at room temperature for 4 hours.
Turn out the set marshmallow onto a work surface dusted with the marshmallow dust and cut the marshmallows into pieces using an oiled knife or a pair of scissors. Toss the marshmallow pieces in the marshmallow dust and store in an air-tight container.
Marshmallow Dust
Mix together sifted icing sugar and cornflour in a small bowl and use as needed.
Notes
My candy thermometer clips to the side of the saucepan but the amount of liquid in the pot isn't enough to cover the part that reads the thermometer. I use a timer and close to the time I would expect the temperature to be right, I tip the saucepan slightly so that the sugar syrup is deep enough for the thermometer to get a reading. I continue to check every minute or so until it's the right temperature.
If you don't have a thermometer, once the sugar boils - the entire surface will be covered in bubbles and the syrup will be clear as the sugar will all be dissolved - start the timer. It will take 7 minutes over a medium heat to reach 115°C.
To make thicker marshmallows, use a loaf tin instead of a 20cm x 20cm tin