Whisk together flour, cocoa powder, baking soda, and salt. Set aside.
Add softened butter and brown sugar to the bowl of a stand mixer fitted with a paddle attachment. Mix on medium speed until light and fluffy, about 5 minutes—occasionally scraping down sides of bowl.
Add egg and vanilla to mixer bowl. Resume mixing on medium speed just until smooth—occasionally scraping down sides of bowl.
With mixer on low, slowly stir in flour mixture until fully incorporated. Stir in chocolate chips.
Cover the bowl and let it chill in the refrigerator for an hour or more before scooping and baking the cookies.
Preheat oven to 350℉. Once the cookie dough is thoroughly chilled, scoop golf-ball sized balls of dough onto a parchment-lined baking sheet. Bake for 8-10 minutes. Cookies will look under done when you remove them from the oven – don’t worry, as they will continue to cook!
Let the cookies cool for 10 minutes before trying to remove them from the cookie sheet. Enjoy immediately after they cool or store in an airtight container for later!