Units Scale

Put the potatoes in a saucepan with 2 litres of cold water and 1 tablespoon of coarse salt. Bring to a simmer, cover, and cook until a knife slips in the potatoes easily and cleanly, about 25 minutes.
Drain the potatoes and peel them. Put them through a potato ricer (or a food mill fitted with its finest disk) into a large saucepan. Turn the heat to medium and dry the potato flesh out a bit by turning it vigorously with a spatula for about 5 minutes.
Meanwhile, rinse a small saucepan and pour out the excess water but do not wipe it dry. Add the milk and bring to a boil.
Turn the heat under the potatoes to low and incorporate the well-chilled butter bit by bit, stirring it in energetically for a smooth, creamy finish. Pour in the very hot milk in a thin stream, still over a low heat, still stirring briskly. Keep stirring until all the milk is absorbed. Turn off the heat and taste for salt and pepper.
For an even lighter, finer purée, put it through a very fine sieve before serving.