Preheat the oven to 170°C (338°F).
Sift the matcha powder and flour into a bowl and mix together.
In another bowl, cream the butter and sugar together.
Add the eggs and mix well.
Add the matcha-flour mixture and fold with a spatula.
Mix in dark chocolate or nuts, if desired.
Pour the batter into a pan and level the surface.
Bake in the oven at 170°C (338°F) for 20–25 minutes.
Let it cool and cut into desired sizes.