Whisk the starter, milk, water and sugar together with a fork.
Add the flours and salt. Stir with the fork until a stiff dough forms then finish by hand. Cover and let rest 30 minutes. Knead four or five times in the bowl to bring the mixture together into a smoother ball.
Cover and let rise 8-10 hours at room temperature (68-70 degrees).
Line a sheet pan with parchment paper dusted with cornmeal.
Turn the dough out and divide into 4, 240-gram balls.
Turn the ends under and shape on an unfloured surface into a ball.
Transfer to the sheet pan; cover with greased plastic wrap and rise until noticeably puffy. About 1-2 hours.
Preheat the oven to 400.
Using a sharp knife or razor blade, score the top of each roll 1 1/2 inches deep right across the center.
Bake on the center rack until the internal temperature reaches 180-200 and the bowls are golden. 35-40 minutes. Tent with foil if they start getting too dark.
Once cool enough to handle, cut a shallow circle in the top and hollow out the inside of the bowl with your fingers, leaving at least 1/2" of bread on the inside to prevent leaks.