Great mashed potatoes are all about contrast, with a cloud-like, almost airy texture that plays against the deep, buttery flavor. Using russet potatoes is the key to lightness, while adding both butter and heavy cream makes this version especially rich. Rinsing the potatoes — as recommended by Kenji López-Alt — keeps them soft and supple, while using a potato ricer or food mill helps the boiled potatoes break down smoothly and introduces air, which fluffs them up. (Add a little cream or milk if you want a looser texture.) And yes, you can double this recipe and make it up to five days ahead; see Step 10 for tips on reheating.