To make this the first time - Use 10 grams of an active regular sourdough starter that you maintain. Mix the starter with the rest of the ingredients in a jar using a spatula until thoroughly incorporated. Cover slightly and allow to ferment at room temperature until ready to use. It should be ready within 12 hours.
To maintain the starter - Discard as you would a regular starter, use it to bake bread or make a sourdough discard treat. Use 10 grams of your chocolate sourdough starter, mix it with the rest of the ingredients as in step 1. Let it ferment at room temperature or in the refrigerator to lengthen time between feeds.
Notes
You can keep this starter in the fridge and feed it on a weekly or bi-monthly schedule. It does not need to be fed everyday. See post for more detailed maintenance tips.