Pre heat oven at 250 degrees.
Cream butter and sugar together.
Slowly add in egg yolk and beat till light and fluffy.
Mix in the, white oats, whole wheat flour, wheat bran, cocoa powder and salt.
Shape the dough into even sized balls and flatten slightly.
Sprinkle some oats on top and bake in a pre-heated oven for 10 minutes or till done at 160 degrees C.