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1
Place the peeled, uniformly cut potatoes in a pot of cold, salted water. Bring to a gentle boil and simmer for 20–25 minutes. The dish is ready to move forward when a knife slides through a piece without resistance but the potatoes hold their shape and do not crumble.
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2
Drain thoroughly. Return the potatoes to the dry pot over low heat and shake the pot for 2–3 minutes. This is where the dish lives or dies: the surface of the potato must turn visibly chalky and dry, with no trace of steam rising from moisture — any residual water will make the finished purée waterlogged and gluey, a fault no quantity of butter can correct.
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3
Pass the hot, dry potatoes through a potato ricer or food mill fitted with its fine disc — twice if following the Robuchon standard. A food processor or blender must never be used; the blades rupture the starch granules and produce an irreversible wallpaper-paste consistency. The riced potato should be light and visibly fluffy.
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4
Return the riced potato to a clean pot over low heat. Begin adding the cold butter a few cubes at a time, stirring vigorously with a wooden spoon or spatula after each addition. Continue until all 250 g are incorporated. The mass should become smooth and glossy as the butter emulsifies into the hot starch.
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5
Gradually add the warm milk in a thin stream, stirring continuously, until the purée flows lazily from a spoon — it should move with a slow, liquid ease, not stand in stiff peaks. Season with salt and white pepper.
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6
For the Robuchon finish, pass the completed purée through a fine tamis (drum sieve). The result should be pourable and absolutely smooth to the eye and palate.
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7
Serve immediately. If holding, place the pot in a bain-marie at 65 °C and dot the surface with a knob of butter. The purée will hold for up to 30 minutes; the butter surface, still glossy and unbroken, confirms it has not skinned over.