Place potatoes in a Dutch oven or a large saucepan with enough lightly salted water to cover. Bring to boiling. Reduce heat, cover, and simmer for 20 to 25 minutes or until potatoes are tender when pierced with a fork. Drain. Mash with a potato masher, or beat with an electric mixer on low speed. Add margarine or butter, sour cream or yogurt, salt, and white pepper. Gradually beat in enough milk (about 1/2 cup) until potatoes are light and fluffy. Serve immediately. Makes 10 to 12 servings.
Spoon mashed potatoes into a buttered 3-quart casserole. Cover and refrigerate up to 2 days. To reheat, cover and bake in a 350 degree F oven for 1 hour. Uncover, dot with 1/4 cup margarine or butter, and bake 15 minutes more or until heated through. Before serving, sprinkle with 1 tablespoon snipped fresh parsley.