Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a mixing bowl, cream together the butter and sugars until light and fluffy.
Beat in the egg and vanilla extract.
Sift in cocoa powder, flour, baking soda, and salt. Stir until just combined.
Fold in chocolate chips if using.
Scoop tablespoon-sized dough balls onto the baking sheet, spacing them out slightly.
Bake for 8–10 minutes. The centers will look soft—that’s perfect.
Let cool on the sheet for a few minutes before transferring to a wire rack.