Step 1

Peel the potatoes and cut them into 1 1/2- to 2-inch (4- to 5-cm) chunks. Place the potatoes in a large saucepan, add water to cover by one inch (25 mm), and add 2 tablespoons salt. Cover, bring to a boil, lower the heat, and simmer uncovered until the potatoes are very tender when pierced with a small paring knife, 15 to 17 minutes. Drain in a colander and reserve the saucepan.

Step 2

Meanwhile, cut the butter in 1/2-inch (12-mm) dice and put it back in the refrigerator.

Step 3

In a small saucepan over low heat, warm the milk just until it simmers. Turn off the heat.

Step 4

Using a food mill, fitted with the finest blade, set above the large saucepan, process the potatoes into the saucepan. Alternatively, you can use a ricer or good-quality masher to process the potatoes.

Step 5

Set the saucepan over low heat and vigorously whisk in the cold butter several bits at a time, waiting for each addition to be incorporated before adding more butter. When all the butter is added, slowly whisk in enough of the hot milk to make the potatoes the desired consistency—creamy but still thick. You may have some milk leftover. Add 2 teaspoons salt and 1 teaspoon pepper.

Step 6

Whisk in the lemon zest, sprinkle with salt, taste for seasoning, and serve hot.