Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper or a silicone baking mat.
In a medium bowl, whisk together the flour, Dutch-process cocoa powder, baking soda, and salt. Set this mixture aside.
In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar until the mixture is light and creamy, about 2-3 minutes.
Add in the egg and vanilla extract, mixing until everything is well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Make sure not to overmix!
If you’re using chocolate chips, fold them in now.
Scoop tablespoon-sized balls of dough onto the prepared baking sheet, leaving about 2 inches between each cookie.
Bake for 10-12 minutes, or until the edges are set and the centers look slightly soft.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.