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. Slowly add flour mixture to butter mixture. Mix well.
Refrigerate dough for one hour.
Using a cookie scoop, scoop dough and place into granulated sugar, roll around to coat. Place sugar coated dough on lightly greased or parchment lined baking sheet, space cookies 2 inches apart. Bake in a preheated 350 degree oven for 9-10 minutes.
Let cookies cool on baking sheet for 10 minutes before removing to racks to cool completely. Store in an airtight container.