Step 1

Place potatoes in a large pot and pour in cold water to cover by 1". 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).

Step 2

Meanwhile, heat ⅔ cup cream and ⅔ cup 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.

Step 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.

Step 4

Mixing constantly, gradually add warm cream mixture to potatoes, then mix in sour cream. Taste and season with salt and pepper as desired.

Step 5

Do Ahead: Potatoes can be made 2 days ahead. Let cool; cover and chill. To serve, transfer potatoes to a small pot and add ⅓ cup heavy cream and ⅓ cup whole milk; reheat over medium-low, stirring often to prevent scorching.