Boil the potatoes whole, in a large pot of boiling water. When tender, drain them, then rice them with a potato ricer.

While the potatoes are still hot, thoroughly beat them with the margarine, heavy cream, and salt in a large mixing bowl.  Mix in the flour. Roll the lefse dough into plum-sized balls, then arrange on a platter, cover, and refrigerate overnight.

Remove lefse balls from the fridge. While they are still very cold, roll each ball out on a lightly-floured pastry cloth with a lefse rolling pin. Roll each one into a circle 1/8-inch thick. Heat a lefse griddle to 440˚F. Using a lefse stick, transfer the lesfe to the griddle. Allow to bubble, but not to go beyond light brown. Flip onto the other side, and do the same. When finished, use the lefse stick to move the lefse to a slightly damp cloth, and cover. 

When all the lefse sheets are grilled and have cooled, fold into fourths, and store in an airtight container.