Place the chick peas in a casserole, cover with cold water and bring to the boil. Skim, season and add the ham, leeks and bouquet garni. Cook at a slow boil for 2 hours, then add the potatoes and cook for another 30 minutes, until soft.
Rub through a sieve and beat in the butter. Check the seasoning and serve. A split pea purée can be cooked in exactly the same way.