Cream together butter and sugar. Add egg and mix until well-combined.
Add cocoa, flour, and baking powder. Mix well (I recommend using your hands).
Chill for about 15 minutes.
Roll into balls about 1 inch in diameter (depending on how large you would like the cookies).
Add extra sugar to a bowl. Place each ball into the bowl of sugar and roll to coat well. Place each ball onto a cookie sheet, about 2 inches apart. Bake for 9 minutes. Remove to a baking rack to cool.