From America's Test Kitchen Season 9: One Great Thanksgiving
When cooking our Classic Mashed Potatoes, we boil potatoes in their jackets for earthy potato flavor (and peel them while they’re still hot). We don’t mind this somewhat inconvenient method when we’ve got time to spare, but thought an easier alternative was in order.
Cooking potatoes in their skins preserves their earthy flavor and keeps the starch granules from absorbing too much water, thereby preventing gluey mashed potatoes. To give peeled potatoes the same protection, we made two alterations to our usual technique. Steaming rather than boiling the potatoes exposed the potato pieces to less water, reducing the chance of the granules swelling to the point of bursting. When they were cooked partway, we rinsed them under cold water to rid them of free amylose, the substance that results in gluey mashed potatoes, and returned them to the steamer to finish cooking. Because potatoes cooked this way are so full of rich potato flavor, we were able to use less butter and substitute whole milk for cream.
This recipe works best with either a metal colander that sits easily in a Dutch oven or a large pasta pot with a steamer insert. To prevent excess evaporation, it is important for the lid to fit as snugly as possible over the colander or steamer. A steamer basket will work, but you will have to transfer the hot potatoes out of the basket to rinse them off halfway through cooking. For the lightest, fluffiest texture, use a ricer. A food mill is the next best alternative. Russets and white potatoes will work in this recipe, but avoid red-skinned potatoes.