Peel (or scrub) and cut the potatoes into even chunks about 1 to 1½ inches. Even sizing ensures they cook at the same rate.

Place potatoes in a large pot and cover with cold water by about an inch. Add 1/2 teaspoon kosher salt to the water. Bring to a gentle boil over medium-high heat, then reduce to a simmer and cook until potatoes are tender when pierced with a fork, about 15–20 minutes.

While the potatoes cook, warm the milk and heavy cream together in a small saucepan or microwave until hot but not boiling. Stir in the butter so it melts and the mixture is ready to add.

Drain the potatoes well in a colander and return them to the hot pot for 1 minute to evaporate excess moisture. This keeps the mash from getting watery.

Mash the potatoes with a potato masher or ricer until mostly smooth. For the creamiest texture, use a ricer or food mill; a masher gives a more rustic result.

Gradually stir in the warm milk-cream-butter mixture until you reach your desired consistency. Add sour cream if using. Season with the remaining salt (to taste), pepper, and a pinch of nutmeg if desired.

Transfer the mashed potatoes to an ovenproof dish if you plan to reheat in the oven, or to a shallow airtight container for refrigerator storage. Smooth the surface and dot with a bit more butter if you like a glossy finish.

To make ahead: cover the potatoes tightly with plastic wrap (press a layer of parchment or plastic directly on the surface to prevent a skin) and refrigerate for up to 3 days, or freeze for up to 3 months.

To reheat from the refrigerator: place the mashed potatoes in a heatproof bowl set over a pot of simmering water (double boiler) and stir occasionally, or reheat in a 350°F (175°C) oven covered with foil until warmed through, about 25–30 minutes. Add a splash of warmed milk or cream and stir to refresh creaminess.

To reheat from frozen: thaw overnight in the refrigerator, then follow the refrigerator reheating method. For direct oven reheating from frozen, cover and bake at 350°F (175°C) for about 45–60 minutes until hot throughout, stirring once halfway and adding warmed milk if needed.

Before serving, taste and adjust seasoning, then sprinkle with chopped chives and a final pat of butter if desired. Serve warm and enjoy!