Preheat oven to180°C.
Spray a large baking sheet with cooking spray.
Sift cocoa in a bowl, then add the brown and white sugar, eggs, oil and vanilla extract, whisk together until well combined.
Sift the flour, baking powder and salt in a separate bowl and add to the cocoa mix. Stir through until just combined. Do not over mix.
Stir through the chocolate pieces.
The dough will be a bit sticky, but let it rest in the fridge for at 2-4 hours.
Place the icing sugar in a shallow bowl. Working with about 1 heaped tablespoon of dough (or a medium ice cream scoop) at a time, roll into balls.
Drop dough balls directly into icing sugar and roll to coat.
Evenly space the dough balls on the prepared baking sheets.
Bake the cookies, until puffed and cracked and edges have begun to set but centres are still soft (cookies will look raw between cracks and seem underdone), about 10-12 minutes.
Let it cool completely on sheet before serving. The cookies can be stored at room temperature in an airtight container for up to 5 days.