Preheat oven to 350° F. Line baking trays with parchment paper or silicone baking mats.
In a small mixing bowl, combine flour, dark cocoa powder, baking soda, and salt. Whisk to combine.
In a separate large mixing bowl, cream together butter, granulated sugar, and brown sugar using an electric hand mixer until light and fluffy.
Add in egg and vanilla extract and mix on medium speed until the mixture is light and fluffy.
Add in ⅓ of the flour mixture to the butter, mixing on low speed, and then ½ of the buttermilk, continuing to mix.
Once combined, add another ⅓ of the flour mixture and continue to mix, and then the remaining buttermilk.
Add in the remaining ⅓ of the flour mixture and mix until fully combined. Scrape the bottom and sides of the bowl to ensure the dry ingredients are fully incorporated.
Portion the batter using a large cookie scoop (3 tbsp) onto prepared baking sheet pans. Space the batter out as the cookies spread when baked.
Bake for about 10-12 minutes until the cookies are puffed up.
Remove from the oven and allow the cookies to cool on the tray for 2 minutes.
Remove the cookies from the tray and transfer to a wire cooling rack to cool completely.