Preheat the oven to 180°C (356°F).
In a bowl, mix cake flour, oatmeal, baking powder, sugar, and salt.
Add cold butter, cut it into small pieces with a scraper, and then mix with your hands until crumbly.
Add egg, milk, and green tea powder, and mix to form a dough.
Gather the dough into a ball and flatten it.
Cut into triangles or your desired shapes.
Place on a baking tray lined with parchment paper and brush the tops with milk.
Bake in the oven at 180°C (356°F) for 20 minutes.