Boil the potaotes in their skins in very well-salted water. (Choose roughly similar-sized potatoes so they cook at the same rate.)
When they are thoroughly cooked, drain them in a colander and leave them to steam until just cook enough to handle. Then the skins are easilly peeled off with your fingers.
Break up the potatoes roughly with a fork to allow more moisture to steam off.
Add the milk and butter to a saucepan and heat. Infuse some fresh ground black pepper and a pinch of nutmeg.
After a couple of minutes more steaming, the potatoes can then be sieved into the pan of bubbling hot milk and butter, or even cream and butter and whisk until smooth.
Taste and carefully adjust the seasoning again before serving.