Peel the potatoes, then cut the potatoes into large chunks or slices. Place the potatoes in a stock pot and cover with water. Bring to a boil, then reduce the heat to medium and boil gently until the potatoes are tender, about 15-20 minutes. Drain the water.
In a large bowl, combine the boiled potatoes, butter and salt. Mash the potatoes until smooth. If you have a potato ricer, use it, otherwise, mash by hand, trying to get rid of most of the potato lumps. Stir in the milk.
Set the bowl in the refrigerator, uncovered, and chill for about 3 hours, until cool.
Add 1 cup of the flour, then gradually add the remaining cup of flour, kneading the dough by hand for several minutes to incorporate all the flour.
Divide the dough into 16 equal portions, then shape each portion into a ball.
Heat a 10 or 12 inch non-stick skillet over medium heat until hot. Do not add any oil or cooking spray to the skillet; you'll be cooking the tortillas over dry heat.
Generously flour a clean work surface and roll out one of the balls of dough thinly, to about 8 inches in diameter. You need to use plenty of flour underneath and on top of the dough, or it will stick terribly and you won't be able to transfer the tortilla to the skillet.
The uncooked dough tears easily, so gently transfer the tortilla to the hot skillet. Cook for 2-3 minutes on one side until golden brown, then flip and cook on the other side for another 2-3 minutes until browned. If the tortilla browns too quickly before the dough is cooked through, lower the heat a little.
Transfer the cooked tortilla to a plate, and brush the excess flour off the tortilla and out of the skillet.
Repeat the last few steps until you've cooked all the lefse.