Place potatoes in a large saucepan and cover with cold water by about 1 inch. Add a large pinch of salt.

Bring to a boil over high heat, then reduce to a simmer. Cook until potatoes are very tender when pierced with a fork, about 12 to 15 minutes.

While potatoes cook, heat milk and butter together in a small saucepan over low heat until butter melts and milk is warm. Do not boil. Alternatively, microwave the mixture until warm.

Drain potatoes well in a colander and return them to the hot pan. Let them sit for a minute to dry out slightly.

Using an electric mixer on low speed, start to whip the potatoes while slowly adding the warm milk and butter mixture. Increase speed to medium and whip until light and silky, about 1 to 2 minutes. Avoid whipping too long to prevent gluey texture.

Mix in sour cream, salt, and pepper. Taste and adjust seasoning. If you prefer looser potatoes, add a splash more warm milk.

Transfer to a serving bowl and sprinkle with chopped chives or parsley if using. Serve warm.