Peel the potatoes and cut them into evenly sized pieces about 1½ inches (4 cm). Place them in a saucepan with the cold water and 1½-2 teaspoons kosher salt, then bring the water to a gentle boil.

Reduce the heat and simmer the potatoes for 20-25 minutes, or until they are completely tender and a knife slides through easily. While the potatoes cook, warm the milk gently without allowing it to boil.

Drain the potatoes thoroughly, return them to the warm pan, and set the pan over low heat for 1-2 minutes. This evaporates excess moisture and helps the finished purée remain rich rather than watery.

Pass the hot potatoes through a food mill or potato ricer. Work while they are still hot for the finest texture, and avoid a food processor or blender because over-processing can release starch and create gluey potatoes.

Return the potato purée to low heat and gradually beat in the cold butter a few cubes at a time. Allow each addition to emulsify into the potatoes before adding the next pieces.

Slowly mix in the warm milk until the pommes purée is creamy and smooth. Season with fine salt and, if desired, a small amount of white pepper.

For an especially refined texture, pass the finished purée through a fine-mesh sieve. Serve immediately while the potatoes are warm, glossy, and at their most delicate.