Dissolve the yeast in 5 ozs of warm milk. Put the flour in a terrine or salad bowl, add the dissolved yeast, work into a dough, neither very stiff nor yet too soft. This is entirely a matter of judgment and experience. Let stand for about ½ an hour, and if the dough has risen sufficiently, add the rest of the milk, the salt, and work till the dough no longer clings to the hands. Now roll out on a pastry board and coat with the butter, as in making puff pastry. Fold the dough, shape into a ball and let it stand in a cool place for several hours or even overnight. Roll out the dough, cut in 4 or 5 inch lengths, roll each piece into a cylindrical shape and let stand for 10 minutes. Flatten them slightly and shape into crescents. Put them on a cloth and let them rise for another hour. Then brush over with the yolk of egg and bake in a moderate oven.