Place rice and warm water mixture in a flameproof casserole and leave in a warm place for at least 7 hours.
(Note: Those with severe milk allergies can use lemon juice or vinegar in place of whey, yoghurt, kefir or buttermilk.)
Bring to a boil, skim, reduce heat, stir in salt and butter and cover tightly, Without removing lid, cook over lowest possible heat for about 45 minutes.
