Peel a thin strip around the center of each potato. Place the potatoes in boiling water to cover. Cook until very tender (time will vary according to size of potatoes). When potatoes are done, remove from water and holding each potato on each end with a towel, slip off skins. Mash with potato masher or potato ricer. Drain water from pot in which potatoes cooked, place pot over low heat and add mashed potatoes.
While potatoes are heating and drying, combine the two milks in a saucepan over low heat. Warm for a minute or so.
Add most of the combined milks to the mashed potatoes. Whip rapidly with a wire whisk. Add more milk if a thinner consistency is desired. Season to taste with salt and pepper. If using goat cheese, stir in gently with a fork. Serve immediately.