Place potatoes in a large saucepan; add water to cover and bring to a boil. Reduce heat; cook, uncovered, until tender, 15-20 minutes. Drain. , In a large bowl, beat cream cheese and sour cream until smooth. Add potatoes; beat until light and fluffy. Beat in butter, seasonings and, if desired, chives. Transfer to a greased 2-1/2-qt. microwave-safe dish. Refrigerate, covered, overnight., Remove from refrigerator 30 minutes before heating. Microwave, uncovered, on high 10 minutes, stirring once. Stir in milk. Microwave until heated through, about 5 minutes.