Cling a shallow baking tray, about 30 x 20cm and dust it with sieved icing sugar and cornflour.
Soak the gelatine in 140ml cold water.
Put the sugar, glucose and water into a heavy-based pan. Bring to the boil and continue cooking for about 12-15 minutes until the mixture reaches 127C on a sugar thermometer.
When the syrup is up to temperature, carefully slide in the softened gelatine sheets and their soaking water. The syrup will bubble up so take care not to burn yourself. Pour the syrup into a metal jug.
Whisk the egg whites until stiff, preferably with an electric whisk in a mixing bowl. Continue whisking while pouring in the hot syrup from the jug. The mixture will become shiny and start to thicken. Add the vanilla extract and continue whisking for about 5-10 minutes, until the mixture is stiff and thick enough to hold its shape on the whisk.
Spoon the mixture into the prepared tin and smooth with a wet palette knife if necessary.
Leave for at least 1 hour to set.
Dust the work surface with icing sugar and cornflour. Loosen the marshmallow around the sides of the tray with a palette knife, and then turn it out on to the dusted surface. Cut into squares and roll in the sugar and cornflour. Leave to dry a little on a wire rack.
This is a james Martin recipe. The best marshmallow recipe I've made. From the book, Digs deep.