Stir butter, sugar and vanilla to a smooth batter. Add the oil gently while stirring. Crush the oat and mix with the batter. Add sifted cacao, baking powder and flour. Make a dough.
Make small balls and put a bit apart on a grease proof paper. Bake about 15 minutes in 150°C. Put the cookies on a cooling rack and put in a tin when cooled.