Make-ahead mashed potatoes
Ingredients
1× recipe

4 pounds medium Yukon Gold potatoes, scrubbed
⅔ cup heavy cream plus ⅓ cup for reheating
⅔ cup whole milk plus ⅓ cup for reheating
1¼ cups (2½ sticks) unsalted butter, cut into pieces
½ cup sour cream
Kosher salt
Freshly ground black pepper
1½× recipe

6 pounds medium Yukon Gold potatoes, scrubbed
1 cup heavy cream plus ⅔ cups for reheating
1 cup whole milk plus ⅔ cups for reheating
2 cups (4 sticks) unsalted butter, cut into pieces
¾ cup sour cream
Kosher salt
Freshly ground black pepper
2× recipe

8 pounds medium Yukon Gold potatoes, scrubbed
1⅓ cups heavy cream plus 1 cup for reheating
1⅓ cups whole milk plus 1 cup for reheating
2½ cups (5 sticks) unsalted butter, cut into pieces
1 cup sour cream
Kosher salt
Freshly ground black pepper
Directions
  1. Place potatoes in a large pot and pour in cold water to cover by 1 inch. Add a large handful of salt (water should taste briny, like the ocean) and bring to a boil. Reduce heat and simmer until potatoes are very tender but not crumbly, 30–40 minutes. Drain and return potatoes to warm pot to dry (off heat).
  2. Meanwhile, heat cream and milk in a small saucepan over medium until just about to simmer. Reduce heat to low and keep warm until ready to add to potatoes.
  3. Pass hot potatoes and butter through ricer into a large bowl (this will help combine them quickly; most of the skins should stay behind but pick out any small pieces that get through if you want); season generously with salt. Mix with a potato masher until butter is melted and combined.
  4. Mixing constantly, gradually add warm cream mixture to potatoes, then mix in sour cream. Taste and season with salt and pepper as desired.
Potatoes can be made 2 days ahead. Let cool; cover and chill. To serve, transfer potatoes to a small pot and add extra heavy cream and whole milk; reheat over medium-low, stirring often to prevent scorching.

From Bon Appétit.
The basic principles of all the best mashed potatoes recipes still apply: Use Yukon Golds, not starchier russet potatoes, and don’t peel them before cooking. Leaving the skins on while they simmer prevents the spuds from becoming waterlogged. Process the cooked potatoes through a ricer in batches to avoid lumps and render spuds so smooth, you can just stir everything together, no electric mixer required. And be sure to combine the butter with the hot tubers before adding the cream, milk, and sour cream, which will allow the butterfat to coat the potato starches and prevent the dish from getting sticky or gummy. Once the mashed potatoes have cooled to room temperature, refrigerate them in an airtight container or a well-wrapped casserole dish or other baking dish.

When you’re ready to serve, gently reheat the potatoes with a little more dairy and they’ll be as creamy as if you had whipped them only moments ago.