Preheat the oven to 350°F (180°C), line 3 baking sheets with parchment paper and set aside. In a large bowl, whisk together the flour, cooca powder, baking soda and salt; set aside
In a separate bowl, cream together the butter, brown sugar and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until well combined. Add the flour mixture to the butter mixture and stir until just combined.
Divide the dough using a medium cookie scoop into 20 equal sized dough balls ( I used the medium OXO cookie scoop, 1.5 tbsp size and each ball was about 24-25 grams).
Bake in the preheated oven for 9 minutes, or until the cookies have spread and are slightly puffed. Allow to cool for 5 minutes then transfer to a wire rack to cool completely.