Place 4 pounds cubed Yukon gold potatoes in a large saucepan, cover with water. Bring to a boil. Reduce heat, and simmer 10 minutes or until tender, drain. Return potatoes to pan, add 1/2 cup low-fat buttermilk, 1 teaspoon salt, and 1/4 teaspoon black pepper. Mash with a potato masher until desired consistency.