In a large saucepan, boil cabbage until tender; remove and chop or blend well. Set aside and keep warm. Boil potatoes until tender. Remove from heat and drain.
Chop leeks, green parts as well as white, and simmer them in just enough milk to cover, until they are soft.
Season and mash potatoes well. Stir in cooked leeks and milk. Blend in the kale or cabbage and heat until the whole is a pale green fluff. Make a well in the center and pour in the melted butter. Mix well.