Robuchon Mashed Potatoes (Pommes Purée)

Steps

1. Scrub the potatoes under cold water without peeling them. Put them in a large pot, pour in the 3 l of cold water, which should cover them by at least 3/4 inch (2 cm), and add the coarse salt.
2. Bring to a simmer over medium heat, then adjust the heat to keep a gentle simmer, with no big bubbles that would split the skins. Cook for 25 to 30 minutes: the tip of a knife should slide through the largest potato with no resistance at all.
3. While they cook, cut 380 g of the butter into 3/4 inch (2 cm) cubes and put them back in the fridge: the butter must be very cold when it goes in. Save the remaining 20 g for serving. Pour the milk into a small saucepan, bring it just to a simmer, then keep it warm over very low heat.
4. Drain the potatoes and peel them while they're still piping hot: spear each one on a fork and pull the skin off with a paring knife, or rub them in a clean kitchen towel. Don't let them cool, since cold starch turns gluey.
5. Rinse and dry the pot. Pass the hot potatoes through a food mill fitted with the finest disc, straight into the pot. Never use a blender or food processor: the blades tear the cells and the mash turns as stretchy as glue.
6. Set the pot over medium-low heat and dry out the purée for 4 to 5 minutes, turning it constantly with a wooden spatula: it steams, pulls away from the sides, and leaves a thin film on the bottom. It should no longer look wet.
7. Turn the heat to the lowest setting. Add the cold butter in 4 additions, working the purée vigorously with the spatula after each one, until the butter is completely absorbed before adding more. The purée gets smoother and glossier as you go. If you see butter beading on the surface, pull the pot off the heat for a few seconds and stir hard.
8. Pour in the hot milk in a thin stream, in 3 additions, stirring the whole time, until the purée falls from the spatula in a heavy, supple ribbon. Season with the fine salt, taste, and adjust.
9. For the finest texture, push the purée through a fine-mesh sieve in small batches with a bowl scraper or the back of a spoon. Return it to very low heat for 1 minute, stirring, to warm it back up.
10. Spoon the purée onto a warm white plate, make a shallow well with the back of the spoon, set the reserved 20 g of butter in the center, and finish with a grind of black pepper. Serve right away.