Add the plain flour, baking powder and salt and mix well till you get a crumbly dough. Leave out the salt if using salted butter.
Lightly knead with your hands to form a dough ball. Cut it into halves.
To one half, mix the almond four and combine. Roll this out between sheets of baking paper into a rectangle of around 1/4 cm thickness.
To the other half, mix the cocoa powder and combine to form the dough. Roll this out to a rectangle of the same size as the white one.
Without peeling the baking papers off, let the two layers chill in the fridge for at least half an hour.
Remove from fridge. Peel off top layers of baking paper. Place the white layer on top of the chocolate one and align.
Carefully, roll along the long end tightly. Use the baking paper to help you along. If the outer dough cracks, let it soften for a few minutes and roll. Wrap with the baking paper and leave it in the fridge for 3-4 hours at least or overnight. The dough has to firm up well.
Slice into 1/2cm thick slices and bake in a 180C preheated oven for 10 minutes.