1. We left the room temperature butter to soften. We work the softened butter with icing sugar and vanilla.
  2. Add the egg and mix until dough is smooth. Add the flour and mix until a ball, without overworking the dough.
  3. We separate the ball in two. We incorporate the sifted cocoa in half, and keep the two masses in the cold for 15 minutes.
  4. Roll out the dough leaving it white 2-3 mm thick and rectangular shaping (21 x 15 cm), and cocoa with dimensions somewhat lower than the previous (20,5 x 14,5 cm). Place the rectangle on the cocoa vanilla and rolled over with no space between the two masses. Coat the roll in white sugar.
  5. Cut slices of approximately 3 mm thick and arrange them on a sheet of greaseproof paper. Bake at 180 ° C until they become slightly discolored (of 15 a 20 minutes).

I commented a trick to facilitate the work: use an A4 sheet folded in half to form rectangles. This recipe is based on a book proposal “Chocolate recipes”. You enjoy it!