Bake the potatoes, and, when they are done, cut off the tops and scoop out the meat into a hot bowl; mash very fine, and add the butter, the milk and salt and pepper. Some add a little grated Gruyere cheese; this is a matter of taste. Beat all till very light, and add the beaten whites of two eggs, and stir lightly. Fill the Potato skins with this preparation, heaping the flaky Potatoes nicely on top. Brush over lightly with the yolks of the eggs, set in the oven to brown and serve hot.