This would be the easiest dish in the world were it not for the slight difficulty of judging the consistency of the rhubarb. It should be sludgy, not runny; and that usually means that you have to reduce the liquid it throws off.
The quantities below served 4 people. The rhubarb sticks were thick, and the volume of cream was about a third of that of the rhubarb.
6 sticks rhubarb
1 1/2tbsp caster sugar (or more, or less, according to taste)
284ml double cream
1 tsp vanilla essence
Wash the rhubarb, cut it into short lengths, and throw it into the pan with 50ml water. You need the water to stop it catching, even though you'll find that you have an abundance of liquid later. Cover the pan, and cook over a medium heat. Add the sugar after a few minutes. (If you put the sugar in right away, it might catch on the pan and caramelise.) Turn down the heat once the contents of the pan are cooking properly. Allow the rhubarb to soften - about 10 to 15 minutes.
Drain the rhubarb in a sieve over another pan. Don't throw away the liquid it sheds: boil and reduce it, until you have a liquid with a syrupy consistency. Recombine with the rhubarb, mix it up into a mush, and cool. Add more sugar, if you like.
Whisk the cream, with a balloon whisk, for a minute or so, until it stands in peaks. Be careful: if you go on too long, the cream will get very stiff. Fold the cream into the rhubarb, with the vanilla.
Divide the fool between four glass bowls, cover with cling film, and chill.