Preheat the oven to 350°F/180°c. Line 2 baking sheets with parchment paper and set aside.
Add the cold butter, brown sugar and granulated sugar to the bowl of a stand mixer. Beat on a medium speed using a flat beater attachment for around 3 minutes, or until creamy.
Add the egg and vanilla extract to the stand mixer bowl, and beat on a low-medium speed for 1 minute, or until combined, scraping the sides of the bowl as necessary.
Add the all-purpose flour, cocoa powder, baking soda and salt to the stand mixer, and mix on a low speed until a dough forms and no white streaks of flour remain.
Fold in the chocolate chips.
Divide the dough into 12 portions. Roll roughly into balls, and place on the prepared baking sheets, leaving a space between each dough ball. Press extra chocolate chips on top of the dough balls, if using.
Bake the cookies for 10–12 minutes, or until the outside of the cookies are baked, but the cookies are still soft. Allow to cool on the baking sheets for 5 minutes, before transferring to a cooling rack to cool completely.