Pass hot potatoes and 1¼ cups (2½ sticks) unsalted butter, cut into pieces, 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. Mixing constantly, gradually add warm cream mixture to mashed potatoes, then mix in ½ cup sour cream. 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 (but gently) to prevent scorching.