From America's Test Kitchen Season 1: Sunday Dinner
Many people would never consider consulting a recipe when making mashed potatoes, instead adding chunks of butter and spurts of cream until their conscience tells them to stop. Little wonder then that mashed potatoes made this way are consistent only in their mediocrity. We wanted mashed potatoes that were perfectly smooth and creamy, with great potato flavor and plenty of buttery richness every time.
We began by selecting russet potatoes for their high starch content. Through trial and error, we learned to boil them whole and unpeeled—this method yielded mashed potatoes that were rich, earthy, and sweet. We used a food mill or ricer for the smoothest texture imaginable, but a potato masher can be used if you prefer your potatoes a little chunky. For smooth, velvety potatoes, we added melted butter first and then half-and-half. Melting, rather than merely softening, the butter enables it to coat the starch molecules quickly and easily, so the potatoes turn out creamy and light.
Avoid using unusually large garlic cloves, which will not soften adequately during toasting. Yukon Gold, red, russet, or white potatoes can be used--each turns out a different texture. For smooth mashed potatoes, a food mill or potato ricer fitted with the finest disk is the best choice. For chunky mashed potatoes, use a potato masher, decrease the half-and-half to 3/4 cup, and mash the garlic to a paste with a fork before you add it to the potatoes.