Chop the peanuts into small pieces (e.g. in a small pot for the hand blender).
Mix the butter, sugar and egg together. Mix the flour with the oat flakes and baking powder, add to the mixture and mix. Add the chopped peanuts and mix everything together.
With 2 spoons or, as I do, with my hands, put about 3 cm small heaps on the baking sheet and bake at 180 degrees for about 10-12 minutes.