In a large bowl using a stand or hand-held mixer, cream the butter and sugars together on medium speed for about 2 minutes until well creamed. Then add the egg and vanilla and continue mixing until combined - about 30 seconds. Turn off the mixer and scrape down the sides of the bowl.
Then slowly add in the flour, cocoa, baking soda and salt mixing on low speed until incorporated. Turn off the electric mixer and stir in the chocolate chips. I usually reserve about ¼ cup of chocolate chips for sprinkling on top.
Form dough into 3 tablespoon balls and place 2 inches apart on a baking sheet lined with parchment paper or a silicon baking mat. Cover with cling film and chill in the refrigerator for at least 2 hours or up to 48.
When ready to bake, preheat the oven to 350F degrees. Bake cookies for 10-12 minutes until the tops are just set. Remove from oven and let cool on their tray for 5 minutes before transferring to a wire rack to continue cooling.