1. Put the blackcurrant juice and sugar into a saucepan and heat until the sugar has dissolved. Leave to cool.
2. Soften the gelatine in the water in a small heatproof container.
3. Stand the container in a pan of hot water and stir until dissolved, then add to the saucepan.
4. Pour into a basin and whisk well until thick and foamy.
5. Pile quickly into individual serving dishes and leave for about 45 minutes to set.