In a small bowl, mix the icing sugar with the cornflour and set aside. Pour 150ml of hot water into a large heatproof jug, add the gelatin pieces, stir together and leave to melt. Lightly grease and line a 23cm x 23cm square tin with nonstick baking paper and dust with some of the icing sugar mix. Pour 200ml of water into a medium sized pan, add the granulated sugar and liquid glucose and stir over a low heat until the sugar has dissolved completely, turn up the heat and boil until the mixture reaches firm ball stage on a sugar thermometer (125°C/256°F) - around 10-15 mins.
If you don't have a thermometer, drop a little of the mix into a glass of very cold water - if it sets into a soft ball, it is ready. If not place it back onto the heat and keep heating until the firm ball stage is reached. While the sugar is boiling, use an electric whisk to beat the egg whites until stiff. When the syrup reaches the right stage, pour it into the jug with the melted gelatine and water; be careful as it will be very hot and will bubble up. Continue whisking the egg whites while pouring on the syrup in a steady stream, then add the vanilla extract - the mixture will go shiny and start to thicken.
Continue whisking for about 10 minutes or until the mixture is very stiff. Set aside and leave somewhere cool and dry to set for at least 3-4 hrs or until firm and not sticky to touch. Lay a large sheet of nonstick baking paper on a chopping board and sprinkle on the remaining icing sugar mix. Turn the marshmallows out, dust in the sugar mix and cut into 3cm x 3cm squares. Keep in an airtight container and although they are best eaten the day they are made, they will keep for a few days in a sealed container.
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