Fresh fava beans and buttermilk add texture as well as flavor these mashed potatoes.
Place potatoes in a small stockpot, cover with water, and bring to a boil. Reduce heat to medium high, and gently boil until tender, about 40 minutes. Drain in a colander. Using a towel to hold the potatoes, peel while still hot. Pass them through a potato ricer or a food mill into the top of a double boiler or a large heatproof bowl. Meanwhile, fill a small stockpot or bottom of a double boiler about a quarter of the way, and bring to a simmer.
To the riced potatoes add the buttermilk, butter, salt, and pepper; mix well to combine. Cover the bowl, and place over simmering water to keep warm.
Meanwhile, split fava-bean pods lengthwise with your fingers, and remove beans. Bring a small saucepan of water to a boil, and add beans. Cook until beans are very tender, about 3 minutes; drain. Peel one end of each bean, squeeze gently to pop bean from skin; discard skins. Roughly mash half the beans, and stir into potatoes. Serve immediately garnished with the remaining whole fava beans.
Frozen lima beans can be substituted for fresh fava beans.