In a large bowl cream together the butter and sugar until smooth and pale. Stir in the eggs one at a time then add in the vanilla.
Sift together the flour, cocoa, baking soda and salt then stir into the creamed mixture.
Drop teaspoonfuls on to ungreased baking trays and bake for 5 minutes until the cookies drop and flatten. Remove briefly and add 4 smarties to each cookie, return to the oven and cook for a further 5 minutes.
Remove the cookies from the oven and cool on the baking tray for 5 minutes then transfer to a wire rack to cool completely.