First, prep the containers: you will need 3 tupperwares roughly about 20cm x 12cm. Brush each container with vegetable oil and set aside. You can adjust the size of the containers depending on how tall you want your marshmallows. You could use one large container of 35 x 20 cm roughly, if keeping these as one flavour.
In a small bowl, mix together the cornflour with the icing sugar. I like to use a sieve for this bit just to make sure they are completely lump free.
Take one of the oiled containers and tip in the icing sugar-cornflour mix, turning to coat the base and all the sides. Tap out any excess onto a non-stick baking tray and reserve for later. Repeat with the other two containers using the icing sugar-cornflour mix. Set aside.
Place the gelatine into a large heatproof bowl (ideally the bowl of a stand mixer if you have one). Add the 2 tbsp cold water and mix well. Set aside.
Next, add the glucose syrup and caster sugar to a large saucepan. Pour in 150ml water and place over a medium heat. Stir gently until the sugar has dissolved in the water. Once the mix begins to simmer, stop stirring. Continue to heat over a medium-high heat, gently swirling the pan, until the sugar syrup reaches 115C (use a jam thermometer to check). At this stage the liquid should be clear and bubbling. Handle the pan carefully as it will be extremely hot.
Remove the pan from the heat and very carefully drizzle the sugar syrup into the gelatine whilst whisking with an electric whisk. You will find this easiest if you use a stand mixer fitted with the whisk attachment.
Once all has been added, whisk on medium-high speed for a further 12-14 minutes - or until you have a thick, white meringue-like texture.
Spoon the mixture into the prepared containers and smooth over the top using a spatula or palette knife. Leave to set for at least 2 hours.
Set out your cornflour-dusted baking tray (the one dusted with the icing sugar-cornflour from earlier). Tip the marshmallows onto the tray. Dust a large sharp knife with icing-sugar cornflour then cut the marshmallow into equal-sized cubes. Toss to coat each one in the icing-sugar cornflour. Enjoy!