Peel potatoes and cut into 2-inch chunks. Place in large saucepan and add enough water to cover by 2 inches. Add 1 teaspoon salt and bring to a boil. Reduce heat to medium and cook 20 minutes, until potatoes are tender. Drain; return potatoes to pot.
Combine milk and butter in glass measuring cup and microwave on high 1 minute, or until butter melts. Pour into potatoes and mash coarsely. Add sour cream, chives, pepper, and remaining salt; mash to desired consistency. (Potatoes will have the best texture if made close to serving time.)