1. In a large bowl combine starter and water until dissolved, stir in flour and mix until no dry bits remain and let rest for 30 minutes.

2. Sprinkle in salt and yeast if using, then knead or stretch and fold the dough until smooth and slightly tacky, about 5 to 7 minutes.

3. Cover and let the dough rise at room temperature for 3 to 4 hours, performing gentle folds every hour.

4. Turn dough out onto a floured surface, divide into four rounds, shape each into a tight ball, and place seam-side down on parchment; let rise 45 to 60 minutes.

5. Preheat oven to 450°F and preheat a Dutch oven or baking stone if using, then score tops with a sharp blade.

6. Bake for 20 to 30 minutes until golden and hollow-sounding when tapped, then cool on a rack.

7. Once cooled, slice off tops and remove inner crumb leaving a 1/2-inch shell for the bowls.