Put the potatoes in an 8-qt. stockpot. Add the salt and enough water to cover the potatoes by about 2 inches. Cover and bring to a boil over medium-high heat. Reduce the heat to medium low and simmer, uncovered, until the potatoes are tender but not falling apart, 15 to 20 minutes.
Meanwhile, heat the heavy cream (or half-and-half or milk) in the microwave or in a medium saucepan over medium heat until hot but not boiling. Keep hot.
Drain the potatoes and return them to the pot over low heat. Add the butter, about two-thirds of the hot cream, and the white pepper. With a potato masher, mash the potatoes to your desired consistency, adding the remaining cream if they seem dry. Season with more salt and white pepper to taste.
To keep them warm, transfer the mashed potatoes to a heatproof bowl, cover them tightly with foil, and set the bowl over a saucepan of barely simmering water.