In 8-quart saucepot, combine potatoes, 1 teaspoon salt, and water to cover; heat to boiling on high. Reduce heat to low; cover and simmer 45 to 55 minutes or until potatoes are tender when pierced with tip of knife. Drain well. Meanwhile, in microwave-safe cup, microwave milk and margarine on High 1 minute or until margarine is melted and milk is warm. Return drained potatoes to saucepot. With potato masher, coarsely mash potatoes with sour cream. Gradually add warm milk; continue to mash until well blended but potatoes are still slightly chunky. Stir in snipped chives, 1 teaspoon salt, and 1/2 teaspoon freshly ground black pepper. To serve, spoon potatoes into serving bowl and garnish with additional snipped chives.