3 pounds russet potatoes, peeled and cut into chunks
2 sticks unsalted butter, at room temperature
3/4 to 1 cup whole milk, warmed
Chopped fresh chives, for topping
Put the potatoes in a large pot, cover with cold water by 1 inch and season generously with salt. Bring to a boil, then reduce the heat to a simmer and cook until the potatoes are tender, 12 to 15 minutes. Drain well.
Press the potatoes through a ricer or food mill back into the pot. (Alternatively, return the potatoes to the pot and mash with a potato masher.)
Fold in all but 2 tablespoons of the butter until melted. Gradually stir in 3/4 cup milk until smooth. If the potatoes still seem thick, gradually add the remaining 1/4 cup milk as needed. Season with salt.
Transfer the potatoes to a serving dish. Top with the reserved 2 tablespoons butter and sprinkle with chives.