1. If using fresh butter beans, cover with plenty of cold water and soak overnight.
2. Drain off the water and cover with fresh cold unsalted water, bring to the boil, cover and simmer for 1 1/2 - 1 3/4 hours, until the beans are quite tender. If using canned butter beans, cook as directed on the can.
3. Drain, then return to the pan and mash until smooth with a potato masher.
4. Push puree to one side of pan, melt butter in other side, then stir into the puree with seasoning. Stir over a low heat until thoroughly hot.
5. Spoon into a serving dish and scatter generously with parsley for serving.