Whisk flour and salt in a large bowl. Mix water, starter, and sugar separately, then combine with flour until a shaggy dough forms. Fold in chocolate chips and rest 30 minutes.

Perform 3 sets of stretch and folds over 2 hours, then bulk ferment at room temperature until doubled.

Preshape the dough, rest, then shape into a tight boule and place seam-side up in a floured banneton. Proof at room temp for 1-2 hours, then cold proof overnight.

Preheat Dutch oven to 500°F for 1 hour. Turn dough onto parchment, score deeply, and bake covered for 20 minutes. Reduce heat to 450°F, uncover, and bake 25-30 minutes until deeply browned.

Cool the loaf completely on a wire rack, at least 1 hour. Slice with a serrated knife and serve toasted with butter.