Mix oatmeal, cake flour, baking powder, and salt together in a bowl.
Soften the butter, add sugar and brown sugar, and mix well.
Add the egg yolk and mix thoroughly.
Add the dry ingredients and combine with a spatula.
Shape the dough into balls and place them on a baking pan.
Bake in a preheated oven at 180°C for 12-15 minutes.