Step by step recipe:

1. 5 min.
   Put into a food processor bowl: 250 g flour, 125 g butter, 125 g caster sugar and 1 egg.
2. 5 min.
   Mix on slow speed until the dough is smooth and even.
3. 5 min.
   Remove half the dough. Add 15 g powdered cocoa to what is left.
   
   Start the motor again and knead until the dough is evenly coloured.
4. 5 min.
   Form the dough into a flat cake and wrap in stretch plastic film. Put in the fridge for 1 or 2 hours.
5. 10 min.
   Put the other half of the dough in the food-proccessor bowl and add 10 g Matcha green tea powder.
   
   Start the motor again and knead until the dough is evenly coloured.
6. 1 hour
   Form the dough into a flat cake and wrap in stretch plastic film. Put in the fridge for 1 or 2 hours.
7. 10 min.
   Then roll out both cakes of dough into 2 large and fairly thin rectangles.
8. 5 min.
   Remove any remaining flour from the dough (with a soft brush, for example) and moisten the top of each rectangle using a pastry brush. This is so that the 2 pieces of dough will stick together.
9. 3 min.
   Place one rectangle on top of the other with the moistened sides together. Press gently with the rolling pin to help them stick.
10. 2 min.
    Moisten the face which is now on top.
11. 3 min.
    Trim the short edge of the rectangle nice and straight to give you a clean start.
12. 2 min.
    Then roll up the dough tightly...
13. 5 min.
    ...right to the end.
    
    Roll under your hands to even up the shape and seal the seam well.
14. 45 min.
    Place the roll on a baking sheet and put in the freezer for 45 minutes. This is to firm up the dough and make it easier to slice.
15. 5 min.
    Preheat the oven to 180°C (360°F).
    
    After 45 minutes, Cut the roll of dough into slices 0.5 or 1 cm thick.
16. 10 min.
    Arrange these on a baking sheet and bake for about 10 minutes.
17. 
    Leave to cool on a wire rack.