Boil the potatoes in their skins for 20-30 minutes. Drain and leave to cool slightly then peel and mash.
Meanwhile simmer the onions in the milk until tender then beat into the mash with a wooden spoon until light and fluffy.
Divide between four plates and make a well in the centre of each. Drop the cubes of butter into each well. As the butter melts take the potato from the outside and dip it into the well of butter.