In a large pot of boiling salted water, cook potatoes, covered, for 20 minutes; drain well and return pan to burner for 30 seconds or until bottom of pan is dry. Mash potatoes with a hand-held masher or a potato ricer until almost smooth, then return cooked potatoes to pan. Mix potatoes with butter, buttermilk, horseradish, salt and pepper. Whip by hand using a wooden spoon until fluffy. Fold in lobster. Taste and adjust seasoning if necessary. Reheats well. Dana’s tip: To cook lobster, bring a large pot full of salted water to a boil. Add a few lemon slices and the lobster. Boil for 8 to 10 minutes or until shell turns pink. Remove from water and let cool enough to handle. Use kitchen shears to cut through underside of tail. Remove meat and chop coarsely.