Preheat the oven: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
- Mix dry ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
- Cream butter and sugars: In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar using an electric mixer until light and fluffy (about 2-3 minutes).
- Add egg and vanilla: Add the vanilla extract and egg to the butter mixture and beat until well combined.
- Combine wet and dry ingredients: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
- Add chocolate chips: Stir in the chocolate chips and chopped chocolate (if using) until evenly distributed throughout the dough.
- Scoop and shape dough: Scoop tablespoon-sized portions of dough and roll them into balls. Place them on the prepared baking sheet, spacing them about 2 inches apart.
- Bake the cookies: Bake for 10-12 minutes, or until the edges are set, but the centers are still slightly soft. The cookies will firm up as they cool.
- Cool the cookies: Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.