In a medium bowl, sift together the flour, cocoa, baking soda and salt. Give the bowl a whisk. Set aside.
In a separate large bowl, beat together the butter, brown sugar and granulated sugar until creamed.
Beat the egg, vanilla extract and peppermint extract into the butter mixture.
Add the flour mixture into the butter mixture. Beat together until the dry ingredients are incorporated. The dough will be very thick and sticky.
Stir in the chocolate chips.
Cover the bowl with plastic and refrigerate for at least 2 hours or up to 48 hours.
When ready to bake, preheat the oven to 350℉ (180℃). Line cookie sheets with parchment paper or silicone baking mats. If the dough has been in the fridge overnight, take it out of the fridge and let it sit on the counter for about 10 minutes so it's a little easier to work with.
Form the dough into balls with about 1 to 1½ tablespoons of dough each - mine weighed about 1 ounce (30ish grams) each. Place 2 inches (about 5 cm) apart on the lined cookie sheets.
Bake 1 sheet at a time in the middle of the preheated oven for about 8-11 minutes or until the tops look just set. For thinner cookies, press the cookie dough balls down slightly before baking.
Cool the cookies on the cookie sheets.