Melt dark cooking chocolate, set aside

Mix butter and sugar until soft, add melted chocolate

Add flour, mix well. Shape the dough into marble-sized balls

Arrange on a flat baking pan, press the middle with the end of your little finger until a small well is formed

Bake in a hot oven (175C) for about 20 minutes

Let cool, then fill in the well in the middle of the cookies with melted chocolate and wait until it hardens