Whisk together the flour, cocoa, baking soda and salt in a medium bowl. Set aside.
In a large bowl, beat together the butter, brown sugar and granulated sugar until combined.
Beat the egg and vanilla extract into the butter mixture.
With the mixer on a low speed, beat the flour mixture into the butter mixture. I typically do this about 1/2 at a time to avoid the dry ingredients from blowing everywhere.
Turn off the mixer and stir in the chocolate chips. I typically reserve about 3 tablespoons of chocolate chips for dotting on top of each cookie later.
Cover the bowl with plastic and refrigerate for at least 3 hours or up to 48 hours.
When ready to bake, preheat the oven to 350F (180C). Line cookie sheets with parchment paper or baking mats.
Form the dough into balls with about 1 tablespoon of dough each (they will be smaller than a golf ball). I use a cookie scoop for this so that I get uniform cookies. Note - if the dough has been in the fridge for over 5 hours or so, you may need to let it sit on the counter for 10 minutes first so that it's easier to work with.
Place dough balls about 2 inches apart on the lined cookie sheets.
Bake 1 sheet at a time in the preheated oven for 9-11 minutes, or until the tops look set.
Remove from the oven. Optionally, dot the top of each cookie with a few extra chocolate chips (if you reserved a few tablespoons). Let cool on the cookie sheet for at least 10 minutes, then transfer to a cooling rack to continue cooling.