In a medium size bowl, cream butter and granulated sugar together until well combined (I've just used a whisk but you can use a stand mixer if you prefer).
Now add the egg and sourdough starter. Whisk together until the batter is light and fluffy.
Now, add the baking soda, salt, cocoa powder and all purpose flour and stir to create a thick cookie dough. The dough will be a bit wetter than a regular cookie dough. The cocoa powder can be a little hard to incorporate, but stick with it, it will come together.
Cover the bowl of cookie dough with a plastic food cover and place into the fridge for at least 2 hours. This dough can be left in the fridge overnight to ferment with no issues.
Preheat the oven to 200C (390F). Prepare two large cookie sheets with parchment paper.
Using a cookie scoop, scoop out balls of dough and roll them into perfect balls. Roll each ball first into the granulated sugar and then into the powdered sugar until they are completely coated and then place onto the prepared cookie sheet. You want to leave a space between each one as they do spread out.
Bake the cookies for 10 to 12 minutes at 200C (390F) or until they are crinkly and just set in the middle. Remove from the oven and allow to cool on the tray for around 10 minutes before transferring to a wire rack to cool completely.
