In a large saucepan cook potatoes, covered, in enough boiling water to cover for 20 to 25 minutes or until tender. Drain potatoes in a colander. Add milk to the same saucepan; heat just to boiling over medium heat. Add drained potatoes, sour cream, mustard, and salt. Remove from heat. Coarsely mash mixture with a potato masher. Stir in chives.