This is not one for children to attempt on their own as it involves boiling sugar but the finished marshmallows are so much yummier than commercial ones, they're really worth having a go. The liquid glucose is not strictly necessary but is an insurance against the sugar crystalising and you having to start all over again.
30g icing sugar
9 sheets gelatine
450g granulated sugar
1 tbsp liquid glucose
2 egg whites
1 tsp vanilla extract
1 Line a 20cm x 30cm tin with baking paper.
2 Sift together the icing sugar and cornflour and sprinkle half into the tin.
3 Put the gelatine into a large glass jug with 150ml water and set aside.
4 Put the sugar, glucose and 200ml of water into a pan and bring gently to the boil, stirring until the sugar dissolves. Boil vigorously for 10 minutes without stirring until the syrup reaches 125C on a thermometer.
5 Meanwhile, whisk the egg whites until stiff.
6 Take the syrup off the heat and let the bubbles subside. Carefully pour the syrup into the gelatine jug and mix well.
7 With the beaters switched on, steadily pour in the hot syrup and the vanilla. The mixture will slacked but keep whisking on full speed (5 - 10 minutes) and once it's nicely thickened, pour into the prepared tin. Leave to set for a couple of hours.
8 Peel off the parchment and cut into squers, rolling them in the reserved icing sugar mixture. Store in an airtight container.