Lightly grease a 20 x 20 cm (8 x 8 in) square baking dish. Line all sides with parchment paper. Grease the paper with more oil and generously sift with the dusting powder.
In a large mixing bowl, add 125 ml (½ cup) of cold water and sprinkle over the gelatine powder. Mix until combined, and set aside to bloom — at least 10 minutes.
To make the sugar syrup, combine the granulated sugar, glucose syrup and water in a small saucepan. Stir together until sugar is moistened.
Set the saucepan over medium heat, and bring the mixture to a gentle boil. From this point onwards, DO NOT STIR. If necessary, gently swirl the pan to ensure even heat distribution. Check for sugar crystals clinging to the sides of the pan. You can swipe these away with a wet pastry brush.
Reduce heat to a low simmer, and clip a candy thermometer to the side of the saucepan. Ensure the tip is submerged in the sugar syrup but not touching the bottom of the saucepan. Continue simmering until your thermometer reads 116℃ or 240℉ (softball stage). Immediately remove saucepan from the heat, and set aside until bubbles have subsided — about 1 minute.
Coming back to your mixing bowl, use an electric mixer on medium speed to break up the bloomed gelatine (it will be chunky and rubbery in texture). While continuing to whisk, slowly pour the sugar syrup down the inner edge of the bowl. Whisk on medium speed for 4-5 minutes, or until the mixture has doubled in size. Turn mixers to high and continue to whisk for another 4-5 minutes, or until mixture is about 3 times its original volume. It should be white and fluffy and leave thick ribbons when the whisks are lifted.
Add salt, vanilla and any other desired colours/flavourings. Whisk until just combined and no streaks remain.
Working quickly, use a greased rubber spatula to scrape the marshmallow into your prepared baking dish (don't worry about getting every last bit). Use the spatula to even out the surface of the marshmallow the best you can. Generously sift with the dusting powder. Cover, and leave at room temperature to set for a minimum of 6 hours or overnight.
Sift a large cutting board with the dusting powder. Lift the marshmallow from the dish and gently peel away the baking paper. Using a greased knife or bench scraper, cut the marshmallow into cubes of your desired size. I like to make mine roughly 3 x 3 cm.