Put some quartered potatoes into a saucepan; just cover with slightly-salted milk, and let them simmer very gently until the potatoes can be mashed easily.
Mash them by stirring frequently with a wooden spoon or small potato masher till they have blended smoothly with the milk.
Do not press them through a sieve.
If the potatoes are slow to cook, you may add a little milk as they are simmering, and then let the liquid reduce again over a gentle heat but the mixture ought in the end to be very thick.
Away from the heat, mix in 6 tablespoonfuls of butter, and serve.