Preheat oven to 150 degrees C

In large bowl, beat butter, sugar, and vanilla until creamy in low speed, do not overbeat. Put in egg and mix. Combine the flour, cocoa, oat, baking soda, and salt; stir into the butter mixture with wood spatula until well blended.

Roll the dough to get ball shape and put onto baking tray.

Press it gently with fork until flat. Take cashew nut, dip it in the egg white and put it on top of the dough

Bake for 25 minutes or until cooked. Move onto wire rack until cool down completely