Combine oatmeal, cake flour, baking powder, and salt in a bowl and mix.
Add green tea powder and sugar, then mix well.
Cut cold butter into small pieces, add to the mixture, and rub in with your hands until crumbly.
Pour in milk and bring the dough together into one lump.
Flatten the dough thickly and cut into triangles.
Bake in a preheated oven at 180°C for 20 minutes.