From America's Test Kitchen Season 4: Steak and Potatoes
Mashed potatoes should be creamy, soft, and supple—but with enough body to stand up to sauce from a stew. As for flavor, the sweet, earthy potato comes first and the buttery richness keeps you coming back for more.
We determined that high-starch potatoes, such as russets, are best for mashing. For mashed potatoes that are rich, earthy, and sweet, it is crucial to cook potatoes whole and peel them after cooking. If potatoes are cut before cooking the result is bland, thin, watery mashed potatoes.
We then turned to the dairy and butter component of our potatoes. Heavy cream tasted too rich, while versions made with whole milk tasted washed out. A combination of half-and-half and butter made for mashed potatoes with a light suppleness and a full flavor. Now that we had decided on the quantities of butter and dairy, we had to determine in what order to add them. We learned that when the butter is added first, the fat coats the starch molecules, inhibiting their interaction with the water in the half-and-half and thereby yielding silkier, creamier mashed potatoes. Also, using melted butter and warming the half-and-half kept the potatoes warm and allows both to be incorporated more quickly.
When it comes to mashing, potato ricers and food mills both produce velvety smooth, fine-textured mashed potatoes. However, we tend to prefer food mills because they have a larger capacity and make for less work. A potato masher is the tool of choice for chunky mashed potatoes, and since they cannot match the smoothness of potatoes from a food mill or ricer, they are best left for coarse and chunky mash.