Step 1

Place potatoes in a large pot and cover with cold water by 2". Generously season with salt and bring to a boil. Reduce heat and simmer until tender, 10–15 minutes. Drain and transfer potatoes to a baking sheet; let stand until dry, 10–15 minutes. Set pot aside.

Step 2

Meanwhile, heat cream, milk, and butter in a medium saucepan over medium heat; season with salt and pepper.

Step 3

Pass potatoes through ricer into reserved pot. Mix in warm milk mixture, then sour cream; season with salt. Serve topped with pepper and more butter, if desired.

Step 4

DO AHEAD: Potatoes can be made 5 hours ahead. Cover and chill. Reheat gently, adding more milk to loosen as needed.