Mix the soft butter with the sugar, starch, flour, and cocoa until you get a crumbly dough.
Add the egg and knead a little to homogenize. If the dough is too soft, add more flour.
Wrap the dough in cling film and refrigerate for an hour.
Remove the vanilla dough from the fridge and roll out into a thin rectangular sheet approximately 0.3cm (0.12 inch) thick.
Proceed in the same way as the vanilla dough.
Put the cocoa dough on top of the vanilla dough, then tightly roll the two layers.
Put the roll in the freezer for 10-15 minutes, until it is harden enough to be easily cut into slices.
Take the roll out of the freezer and cut it into thin slices with a sharp knife.
Place the spiral cookies on a tray covered with baking paper and bake for about 12-13 minutes in a pre-heated oven on medium heat, until they start to brown slightly on the edges.
You should get 2 trays of spiral cookies.
Leave them on a cooling rack to cool, then serve.
Enjoy!