Put the gelatine powder in a small bowl and add 2 tbsp water. Leave to soak for 5 minutes.
Put the rhubarb in a saucepan with 25g of the caster sugar and 30ml water. Simmer gently, stirring for about 10-15 minutes until very soft. Stir the soaked gelatine into the warm water mixture then purée in a blender. Leave to cool.
Stir the cream into the cooled purée mixture and chill for 10-15 minutes until just beginning to set.
Whisk the egg white to soft peaks, add the remaining sugar and whisk until stiff. Fold into the rhubarb mixture and divide between four individual serving dishes.
Stir the cream into the cooled purée mixture and chill for 10-15 minutes until just beginning to set.
1 hour chilling in the fridge is all that’s needed to produce a soft, fluffy mousse. Any longer and the mixture will become too firm.
