Add butter and sugar to a bowl and cream together.
Add the egg and mix well.
Sift in cake flour, baking powder, and salt, and mix evenly.
Add green tea powder and oatmeal, and mix.
Add dark chocolate and walnuts.
Shape the dough into balls and place them on a baking tray.
Bake in a preheated oven at 180°C for 15 minutes.