Build the levain and let it ripen 3 hours. Set the butter out to soften while you wait.
Mix everything but the held-back water, butter, and salt. Add salt and remaining water, then work in the butter. Target dough temp 78°F (25°C).
Bulk 3 hours.
Scale at 85g. Shape into tight balls. Dust ring molds with cornmeal and set one ball in each. No molds? Proof them on a half sheet pan, covered.
Proof on the counter 4 to 5 hours, covered with an inverted sheet pan.
Bake at 480°F (250°C) between two sheet pans: 10 minutes clipped, 5 minutes unclipped, flip, 9 more minutes.
