Peel and cut the potatoes into even 2-inch chunks. Put them in a large pot, cover with cold water by an inch, and add 1 tablespoon salt. Starting in cold water cooks them evenly through.

Bring to a boil, then lower to a simmer and cook 15–20 minutes, until a knife slides in with zero resistance. Drain very well, then return the potatoes to the hot empty pot for a minute to steam off extra moisture — drier potatoes make fluffier, less watery mash.

Rice the potatoes or mash them by hand while hot. Do not use a food processor or beat them hard with a mixer — overworking releases starch and turns them gluey.

Fold in the warm melted butter first, then the cream cheese and sour cream. Add the warm milk or cream a splash at a time until the texture is just right. Season with salt and pepper.

To make ahead: spread into a buttered baking dish, cool, cover, and refrigerate up to 2 days. To serve, dot with a little butter, add a splash of warm cream, cover, and reheat at 350°F for about 30 minutes, stirring once, until hot throughout. Or hold them warm in a slow cooker on low.