- For the vanilla dough: in a mixer combine the flour with the butter and a pinch of salt. Mix until crumbly.
- Beat the egg with the vanilla extract and pour into the mixer together with sugar.
- Mix until all the ingredients are combined. Transfer over a working surface and knead just few seconds.
- For the cocoa dough: put the cocoa and the flour in a mixer. Add the butter and mix on medium speed until crumbly. Add in the egg, yolk and sugar. When the ingredients are combined, transfer over a working surface and shape into a ball.
- Wrap the two portions of dough into the plastic wrap and put in the refrigerator for about 30min.
- Roll the dough into two rectangles and carefully place the cocoa dough over the white one. Cut the extremities to obtain a more accurate rectangle. Roll it up tightly starting from the shorter side.
- Cut the roll into 1/8- inch slices and arrange them over a baking tray lined with parchment paper. Place the tray in the refrigerator for some minutes, until cookies become cold.
- Bake at 180 degrees for about 15 minutes. Let the cookies cool when they are ready, than store in a tin box.