Place potatoes and salt into a pot and fill with enough water to cover. Bring to the boil and cook until potatoes can be easily pierced with a fork. Drain and transfer to a large bowl. Measure in the butter and cream and mash until completely smooth. Cover and refrigerate overnight.
Use your hands to shape the potato mixture into four even logs about 15cm long. Cut each log into 4 or 5 pieces. Using just enough flour to keep the dough from sticking, roll out each portion as thinly as possible. Flip the dough over frequently and add more flour as needed. Adding too much flour can make the lefse tough, so be careful.
Heat flat griddle pan over medium heat. Cook one lefse at a time until golden brown with darker brown bubbles on each side, flipping over once. Use a flat lefse turner if you have one. Stack finished lefse on a plate or tray and cover with a tea towel to keep them from drying out. Once the lefse are completely cool, wrap in cling film to keep moist.