Soak the carrageen in tepid water for about 10 minutes until it softens. Drain off the water and put the carrageen into a saucepan with the milk. Split and scrape the seeds of the vanilla pod into the milk and add the pod too. Bring to the boil and simmer gently for 10 minutes.
Put the egg yolk into a bowl with the sugar and mix well with a whisk. Strain the milk mixture on to the egg y