Put the potatoes in a large pot, add 2 tablespoons salt and enough water to cover the potatoes. Bring to a boil over medium-high heat. Cook until tender, about 20 minutes. Drain well. Return to pan.
Meanwhile, heat the milk and butter in a small saucepan just to warm.
Mash the potatoes, add the milk and butter. Season with salt and pepper. Stir in chives if using.