In your large mixing bowl, use the kitchen scale to accurately measure and then mix together the cocoa powder, sugar, and flour.
Add the established sourdough starter and warm water to the bowl. Use a whisk or wooden spoon to mix all the ingredients until well combined and you have a homogenous mixture.
Pour the mixture into a clean glass jar or container. Ensure there's enough space in the jar for the starter to grow as it ferments.
Lightly cover the jar with plastic wrap or a damp tea towel. If using plastic wrap, ensure it's loose enough to allow air in but will keep out contaminants. Alternatively, you can place a lid loosely on the jar to allow gases to escape.
Place the jar in a warm spot in your kitchen, away from direct sunlight. A top of a refrigerator or inside a turned-off oven with the light on can provide a consistently warm environment.
Allow the mixture to sit for 6-12 hours. Check periodically for signs of activity, such as bubbling and an increase in volume, indicating that the fermentation process is underway.
Once active and bubbly, your chocolate sourdough starter is ready to be used in a variety of chocolate sourdough recipes, from bread to cakes.