Place the potatoes in a large pot, cover with cold water by 1 inch and stir in 1 tablespoon salt.
Bring to a boil, reduce to a steady simmer and cook for 15 to 18 minutes until a knife slides through without catching.
Warm the butter and milk together in a small saucepan until the butter melts; do not boil.
Drain the potatoes well, return them to the hot pot and stir over low heat for 1 minute until the surface looks dry.
Mash the potatoes until no large lumps remain, then fold in the warm milk mixture in three additions.
Fold in the sour cream, remaining 1 teaspoon salt and white pepper just until smooth, then serve immediately.
