Firstly, mix the icing sugar and cornflour in a small bowl. Then line an 8x8inch baking tin with clingfilm and sift over a generous coating of the icing sugar/cornflour mix. Then set aside.
Place the sugar, glucose and water into a small pan and set aside. Place the egg whites into the bowl of a stand mixer fitted with a whisk attachment. And finally, fill a bowl with cold water and set to one side.
Turn the stand mixer onto a medium-high speed and whisk the egg whites until they reach soft peaks. Then place the gelatin leaves into the cold water to bloom, and place the sugar pan onto a medium-high heat.
Cook the sugar until its dissolved, stirring a few times, then turn up the heat and boil until it reaches 127C on a food thermometer. Take off the heat. Quickly, drain the bloomed gelatin, then add the gelatin to the sugar syrup pan (squeezing out any excess water first) and stir to dissolve. The sugar syrup will bubble up. Quickly pour the hot mixture into the whipping egg whites, making sure the syrup is poured directly into the eggs and not down the side of the bowl. It will help to turn the stand mixer down to a medium-low speed at this point.
Then turn the stand mixer up to high, and whip until the mixture has doubled in volume, is glossy and the bowl is cool to touch on the outside. Then add in the peppermint extract and green food colouring. Whip again until combined, then pour into your prepared tin and working quickly, level out. Then sift a generous layer of the icing sugar/cornflour mix overtop and leave to set for around 1-1.5 hours.
Once the marshmallow is set, flip the baking tray over and peel away the cling film. Then cut into squares, making sure to dust each side of the marshmallows as you go.
Marshmallows will keep in an air-tight bag for up to 2 weeks.