1-1/2 lb. Yukon Gold potatoes, peeled and cut into 1-inch pieces
2 Tbs. unsalted butter
2 medium leeks (white and light-green parts only), halved lengthwise, washed, and sliced crosswise into 1/4-inch-wide pieces (about 1 cup)
1/2 cup sour cream, at room temperature
1/2 cup whole milk, heated; more as needed
Freshly ground white or black pepper
Put the potatoes in a large saucepan and cover with cold water by at least 1 inch. Bring to a boil over high heat, add a generous 1/2 tsp. salt, and lower the heat to a steady simmer. Cover the pot partially and cook until the potatoes are just tender when pierced with a fork, 10 to 12 minutes.
Meanwhile, melt the butter in a 10-inch skillet over medium heat. Add the leeks and sauté, stirring often, until tender but not browned, about 6 minutes.
Drain the potatoes and return to the pan. Steam-dry over low heat, shaking the pan until the potatoes leave a light film on the bottom, about 3 minutes.
Mash the potatoes with a potato masher. Stir in the leeks, sour cream, and milk, adding more milk as needed to reach your desired consistency. Season to taste with salt and pepper. Serve.
Variation: Steam-drying the potatoes after they’re cooked allows excess moisture to evaporate, so they’re more intensely flavored and have a denser, more luxurious texture.