Start with the sugar mixture. Bring the water, Muscovado sugar and raspberries to the boil in a saucepan. Boil for about 3 mins.
Pour the sugar mixture through a fine-mesh sieve. Squeeze as much out of the berries as possible. Place the sugar mixture in the fridge to cool fully.
Pour the sparkling wine into a plastic box that holds at least 2 litres and add 200 ml of water and the sugar mixture. Stir well. Put in the freezer for at least 8 hours.
When serving, remove the sparkling wine from the freezer and carve ice crystals by scoring the bubbles with a fork. Scoop the ice crystals into a cold glass and divide between the glasses.
Decorate with lemon balm.
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