Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large bowl, beat softened butter and brown sugar together until light and fluffy, about 2 minutes.
Add the egg and vanilla extract and mix until fully combined.
Add the flour, cocoa powder, baking soda, and salt. Stir until a soft dough forms. Fold in chocolate chips if using.
Scoop dough by heaping tablespoons and roll into balls. Place 2 inches apart on prepared baking sheets.
Bake for 9 to 11 minutes, until the edges are set but the centers still look slightly underdone.
Remove from oven and let cookies cool on the baking sheet for 3 minutes before transferring to a wire rack.