In a large bowl, whisk flour, cocoa, sugar, and salt. Add water and starter, mix until no dry bits remain, and rest 30 minutes.

Pinch in melted butter, vanilla, and chocolate chips. Cover and rest 30 minutes, then perform 3 sets of stretch-and-folds every 30 minutes. Let rise until doubled, about 2–3 hours more.

Shape into a tight round or batard, bench rest 20 minutes, then place seam-side up in a floured banneton. Refrigerate 8–12 hours.

Preheat a Dutch oven at 450°F (230°C) for 30 minutes. Turn dough onto parchment, score, and bake covered 25 minutes, then uncovered 20–25 minutes until internal temp reaches 205°F (96°C).

Cool on a wire rack at least 1 hour before slicing. Serve toasted with butter.