In 3-quart saucepan, place potatoes. Add water just to cover. Heat to boiling; reduce heat to low. Cover and boil gently (simmer) 15 to 20 minutes or until potatoes are fork-tender. Drain.
In same saucepan, mash potatoes, butter and salt with potato masher just until lumpy. Gradually add yogurt, mashing until blended but still lumpy. Stir in pepper and chives.