- STEP 1
In a bowl, mix the icing sugar with the cornflour and set aside. Dissolve the gelatine in 150ml hot water in a heatproof jug. Line a 20cm x 30cm tin with baking parchment and dust with some of the icing sugar mix. Put the granulated sugar and liquid glucose in a heavy-bottomed pan with 200ml water. Cook 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 (125C/256F) – 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 squidgy ball, it’s ready. 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 gelatine water; be careful as it will be very hot and may bubble up.
- STEP 2
Continue whisking the egg whites while pouring on the syrup in a steady stream, then adding the vanilla extract – the mixture will go shiny and start to thicken. Continue whisking for about 10 mins until very stiff. Pour half the egg white mixture into the tin, then scatter over the berries. Top with the remaining mixture and leave somewhere cool and dry to set for at least 2 hrs until firm.
- STEP 3
Lay a large sheet of baking parchment on a chopping board and sprinkle on the remaining icing sugar mix. Turn the marshmallows out and cut into squares, rolling them in the sugar mix. Keep in an airtight container but best eaten on the day.