1. Preheat oven to 350-degree F. 
  2. Mix cocoa, baking soda, salt and flour in a big bowl.
  3. In a separate bowl, with an electric mixer, cream the butter and sugar.
  4. Add flax egg and vanilla.
  5. Add dry ingredients.
  6. Form into 2-inch balls and flatten slightly.
  7. Bake for 9-11 mins. 
  8. Whisk together all frosting ingredients with an electric mixer.
  9. Put into a piping bag.
  10. Once the cookies have cooled, pipe frosting onto one cookie and place another cookie on top to form a sandwich.