Bring milk and garlic to a boil in a small saucepan; remove from heat, and let stand. Meanwhile, place potatoes in a medium pot, and cover with water by 2 inches. Bring to a boil. Cook until tender, 10 to 12 minutes. Drain, and pass through a ricer or a food mill. Return to pot.
Heat potatoes over medium heat, stirring constantly with a wooden spoon, until slightly dried out, about 2 minutes. Strain milk mixture into pot. Discard garlic. Stir in butter. Remove from heat. Stir in yogurt, chives, and 1 teaspoon salt; season with pepper. Garnish with chives. Serve warm.