These creamy mashed potatoes can be made in advance and then stored in the refrigerator or freezer until you are ready to serve them. Slowly heating them in the oven is the perfect way to maintain the fluffiness of the potatoes without making them gluey, which can happen if you heat them to fast and too hot (we're looking at you, microwave).
Lightly scrub the potatoes. Put them in a large stockpot, fill with cold water to cover by about 2 inches and season generously with salt. Bring to a boil, then lower the heat to maintain a strong simmer and cook until the potatoes are fork-tender in the center, 50 to 55 minutes.
Drain the potatoes in a colander in the sink. Let cool enough so that you can handle them, about 10 minutes. It is important that the potatoes do not cool too much because they will mash more easily when hot.
Meanwhile, add the butter to the same pot that you used to cook the potatoes (no need to clean it). Melt completely over medium heat, stirring occasionally, 2 to 3 minutes. Turn off the heat.
Heat 3/4 cup of the cream in a microwave-safe measuring cup or bowl until steaming and set aside.
Holding the potatoes in a clean kitchen towel, gently peel the skins off with a paring knife. Quarter the potatoes and put them in the pot along with 2 1/2 teaspoons salt. Use a potato masher to mash the potatoes until smooth and fluffy (see Cook's Note).
Add the cream and stir until combined. Season with pepper and additional salt if needed. Allow to cool to room temperature, then transfer to an airtight container and refrigerate for up to 2 days or freeze up to 3 months.
To reheat refrigerated mashed potatoes: Position a rack in the center of the oven and preheat to 350 degrees F. Spoon the mashed potatoes into a 13-by-9-inch baking dish. Drizzle with the remaining 1/2 cup cream. Stir until combined, making sure there are no pools of cream at bottom of the baking dish.
Cover with foil and bake for 20 minutes. Remove from the oven and gently stir the mashed potatoes. Cover again and continue to bake until the potatoes are hot and steamy, 20 to 25 minutes more. Season with additional salt and pepper if needed.
To reheat frozen mashed potatoes: Thaw the potatoes completely in the refrigerator and cook according to the same directions as refrigerated potatoes but do not add cream. The potatoes might seem watery at first but will thicken as they heat up.
Transfer the hot mashed potatoes to a large bowl for serving.
If you plan to freeze the potatoes, you will only need 3/4 cup cream total since no additional cream is needed to reheat. For extra-smooth potatoes, use a potato ricer or a food mill instead of a potato masher.
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