Soak the clean, dry carrageen moss in tepid water for 10 minutes. Strain off the water and put the carrageen moss into a saucepan with the milk dessand the vanilla extract. Bring to the boil and simmer very gently with the lid on for 20 minutes.
At that point and not before, separate the egg. Put the egg yolk into a bowl, add the sugar and whisk for a few seconds, then, through a strainer, pour in the milk and carrageen moss mixture on to the egg yolk and sugar mixture, whisking all the time. The carrageen moss mixture will be swollen and exuding jelly, so rub all this jelly through the strainer and into the bowl. Whisk the egg white stiffly and gently fold it into the egg yolk, sugar, milk and carrageen moss mixture in the bowl. It will rise to make a fluffy top. Serve chilled with the poached rhubarb (see previous recipe) and the softly whipped cream. Sprinkle the soft brown sugar on top.