Cut potatoes into 2-inch pieces. In a large heavy saucepan simmer potatoes in salted water to cover by 1 inch 10 minutes, or until tender, and drain in a colander. Return potatoes to the pan and add the butter and 3/4 cup of milk. Using a potato masher, mash potatoes until the desired consistency is attained from lumpy to smooth. Add more milk if necessary. Reheat potaotes over moderately low heat, stirring, and season with salt and pepper.