Preheat your oven to 350°F (175°C).
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a large mixing bowl, beat the softened butter with granulated and brown sugar until creamy.
Add the egg and vanilla extract, mixing until well combined.
Gradually add the dry ingredients, mixing until just combined.
Scoop tablespoon-sized portions of dough onto a lined baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes until edges are set and centers are soft.
Let cool on the baking sheet for 5 minutes before transferring to a wire rack.