Preheat the oven to 370℉/180℃. Line two large baking trays.
In a bowl, use an electric mixer or stand mixer to whisk together your butter and both sugars until fully combined - it should look light and fluffy. Add egg and vanilla and mix again.
Add the flour, cocoa powder, baking powder, baking soda and mix. It should form a thick cookie dough.
Scoop heaped tablespoons of the cookie dough and roll into balls and place on the baking trays, at least 2 inches apart. Press down lightly before baking for 9-10 minutes. Let cool on a wire rack.