Place the sugar, glucose and water in a heavy-based pan. Bring to the boil and continue cooking over a high heat until it reaches 127C/260F on a sugar thermometer - take care as the mixture is very hot.
Meanwhile, beat the egg whites with an electric whisk until stiff peaks form when the whisk is removed.
When the syrup is up to temperature, remove from the heat and 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.
Continue to beat the egg whites while slowly 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 5-10 minutes, until the mixture is stiff and thick enough to hold its shape on the whisk.
Lightly oil a 30cm/12in x 20cm/8in shallow baking tray. Dust the tray with sieved icing sugar and cornflour and then spoon half the marshmallow mixture over and smooth it with a wet palette knife.
Coat the raspberries with a mixture of icing sugar and cornflour, and lay over the top of the marshmallow mixture. Cover the raspberries with the remaining marshmallow mixture and leave for at least an hour to set.
Dust a work surface with more icing sugar and cornflour. Loosen the marshmallow around the sides of the tray with a palette knife and 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 before packing into an airtight box.
Serve the marshmallows as they are or accompanied by melted white chocolate for dipping.
