Put the potatoes, with water to cover, and a dash of salt in a 2-quart saucepan and bring to a boil. Simmer, covered, until the potatoes are soft, 20-30 minutes. Drain well in a colander. Mash to a coarse consistency. Combine the butter, half & half or milk, and sour cream or yogurt and heat in a separate pan. Slowly add mixture to potatoes and mix together with a hand mixer or potato masher until creamy. Add pepper, thyme and salt to taste.