Wearing surgical gloves, steam red and white potatoes with skin still on them, until tender. Remove from steamer and cool. After steamed potatoes cool down, remove the skins and mash really well. Rinse 5 cups of the potatoes and mix with dried potatoes, flour, salt, powdered sugar, butter, and margarine with a pastry blender and knead well.
Preheat an electric, teflon-coated griddle to 500 degrees F. You can also use a large, nonstick pan heated over high heat. Shape dough into golf ball size balls and roll in flour. Cover, and refrigerate for 3 hours, or overnight. Remove balls and slam on well-floured board. Use a notched rolling pin with a cloth cover to flatten and shape balls into very thin circles. The thinner you roll the circles out, the better. Use fresh flour on both cutting board and rolling pin after each ball has been flattened.
Cook flattened balls on the griddle for 8 to 10 seconds on each side. Brush excess flour from lefse after baking and lay between 2 layers of dishtowels. Brush remaining flour off of grill each time. Package in zip lock bags and freeze or refrigerate immediately.