Mashed potatoes seem simple, but we’ve all had the gluey, lumpy, "what went wrong?" version at least once. This method fixes every common mistake, starting the potatoes in salted cold water for even cooking, driving off excess moisture for a lighter texture, and finishing with room-temp butter and warm milk for cloudlike fluffiness. A bright, punchy horseradish-dill compound butter melts over the top, cutting through the richness and turning a simple side into the dish everyone reaches for twice. —Hetal Vasavada, San Francisco, California