Cut the rhubarb into chunks and sweat with the sugar and butter over a low heat until cooked but not mushy. Liquidise or pass through a food mill. When cold, fold into stiffly-whipped cream. Allow to set in the fridge and serve with Lady finger or Boudoir biscuits.
Apple or gooseberry fool is made in exactly the same way, except that in the case of gooseberries the purée should be sieved to remove pips. It may be necessary to adjust sugar to taste.