Melt the butter in a pot without letting it boil. Break the eggs in but set aside 1 white for later. Add the cool milk in which you've dissolved the yeast and sugar.
Stir and gradually add the flour. Knead a soft dough. Leave it to rise. Divide it into 4 balls.
Roll out each ball into a sheet. Smear them with the egg white and sprinkle with sugar. Cut out circles using a cookie cutter. Fold each circle in half and then in half once again, to form seashells. Dip them again in egg white and sprinkle with more sugar.
Arrange the cookies in an oven dish laid out with baking paper. Bake in a preheated oven until golden.