Soak the gelatine in ice-cold water and rinse the sea lettuce.
Heat the sour cream and mix with the lettuce in the blender until smooth.
Add the pre-soaked gelatine and pass through a fine sieve.
Put on ice-cold water.
Beat the whipping cream to yoghurt thickness, add a small portion to the set mixture and mix until smooth.
Fold in the remaining cream and fill the silicone moulds.
Leave to set in the refrigerator and freeze in the blast chiller to at least -20°C to be able to turn them out easily.