Scrub the potatoes well under cold water and, if desired, peel them. Cut into even 1½–2-inch chunks so they cook uniformly.

Place the potato chunks in a large pot and cover with cold water by about 1 inch. Add 1 teaspoon kosher salt to the water.

Bring the pot to a gentle boil over medium-high heat, then reduce to a simmer. Cook until the potatoes are tender when pierced with a fork, about 15–20 minutes depending on size.

While the potatoes cook, warm the milk and butter together in a small saucepan or in the microwave until the butter is melted and the mixture is hot but not boiling.

Drain the potatoes well in a colander, then return them to the warm pot. Allow them to sit for a minute over low heat to let excess moisture evaporate (shake the pot gently to prevent sticking).

Mash the potatoes using a potato masher or ricer until mostly smooth. For the creamiest texture, use a ricer; for a bit more body, use a masher.

Gradually stir in the warm milk and butter mixture, then fold in the sour cream. Mix until the potatoes are silky and you reach your preferred consistency.

Taste and season with additional salt and freshly ground black pepper as needed. Stir in chopped chives if using.

Serve immediately, topped with an extra pat of butter and a sprinkle of chives. If holding before serving, keep warm in a covered dish or in a low oven (200°F/95°C).