Cream together butter and brown sugar with a mixer until fluffy, about 5 minutes.
Add in eggs, one at a time, mixing after each addition, and then vanilla.
Combine cocoa powder, flour, baking soda and salt in a separate bowl. Slowly add flour mixture to butter mixture. Mix well. Cover and refrigerate dough for one hour.
Preheat oven to 350 degrees F.
Scoop dough and form ball. Roll in sugar to coat. Place sugar coated dough on lightly greased or parchment lined baking sheet. Bake for 9-10 minutes.