Preheat oven to 375 F. Cream butter and brown sugar until light and fluffy. Then add vanilla and eggs, then mix well.
Add cocoa powder and carefully mix well. Add flour all at once, then the baking soda and salt. Mix well. (Dough will be stiff!).
Drop by the tablespoonful onto a lightly greased cookie sheet. pat down slightly to form little patties. Dip one side of the patties into granulated sugar and swoosh around a little. The little patties will be very covered in sugar.
Arrange them with a little space between them as they will expand a bit.
Bake at 375 F for 12-14 minutes. Let cool for a few minutes before transferring to a cooling rack or plate.