1. Cut up the pink parts of the rhubarb stem and boil gently in enough water to just cover them. This should take about 10 minutes.
2. Strain the juice in to a jug up to 500ml.
3. Add sugar to taste and then heat gently in a clean saucepan until the sugar has dissolved.
4. Soak the gelatine in cold water for a couple of minutes, and when soft, add to the saucepan and leave on a gentle heat, stirring until completely dissolved.
5. Pour in to glasses and leave to set in the fridge for at about 4 hours.
6. Heat the cream and vanilla extract gently.
7. Whisk the egg yolks until pale in colour.
8. Add the milk to the cornflour and stir until you have a thick paste.
9. Pour the hot cream on to the egg yolks, and whisk until mixed. Add a small amount to the cornflour paste and then pour back in to the cream.
10. Pour everything back in to the pan and return to a gentle heat. Stir slowly, to stop the mixture sticking to the bottom of the pan. Cook for several minutes, until thick, and remove from the heat.
11. Sprinkle with caster sugar or cover with cling film to stop a skin forming and leave to cool.
12. Once the custard is cool enough, fold in the whipped cream, and spoon in to glasses on top of the jelly.