Hard-boil 4 eggs and separate the yolks. Mix the butter with the sugar until whitened.
Add the yolks that you've passed through a sieve and the last raw egg yolk. While stirring constantly, add the flour and knead a soft dough. Put it for 1-2 hours in the refrigerator. Roll out the cooled dough into a sheet with a thickness of 1/4″ (0.5 cm).
Cut out an even number of circles or other shapes, make a hole in the middle in half of them. Bake the dough circles in a greased tray in a preheated moderate oven until ready.