Put the milk, water, butter and salt in a saucepan and bring to a boil. While stirring with a wooden trowel, sprinkle in the sifted flour and keep stirring and moving the dumpling-like dough back and forth in the pot, gently pressing, kneading and "burning off" until a white layer has formed on the bottom of the pot. Then transfer the dough into a bowl, let it cool for about 5 minutes and then stir in the egg and yolk one after the other with the paddle of the hand mixer until the dough is smooth but still a little firm.