Note: For best results, it is important that this dough is chilled before baking. Allow at least 2-3 hours up to overnight if you can.
In the bowl of a stand mixer, using a paddle attachment, cream together the soft butter and sugars on medium speed until smooth.
This can take 2-3 minutes. Make sure there are no lumps of butter.
Add in the sourdough discard and mix until completely smooth.
Use a spatula or wooden spoon to scrape the sides and bottom making sure everything is combined.
Add in the egg and vanilla extract and mix just until combined. Do not over-mix.
In a separate medium bowl combine the flour, cocoa powder, baking soda, and salt. Mix with a fork or whisk to combine.
Add the dry ingredients into the wet ingredients. Adding only a little bit at a time. Mix just until incorporated.
Fold in the chocolate chips.
Cover with plastic wrap and chill in the fridge for 2-3 hours up to overnight.