Instructions

make the sponge

In bowl or container with at least 1-quart capacity, use rubber spatula to stir together starter and water until fully combined. Stir in flour until combined; mixture should resemble thick pancake batter. Cover with plastic wrap and let rise at room temperature (about 72 degrees) until doubled in bulk, 2 to 3 hours.

make the dough

Measure water into bowl of standing mixer; add sponge to water. Fit mixer with dough hook; with mixer running on lowest speed, add flour 1/2 cup at a time. Once all flour has been added, continue kneading until dough forms ball, about 1 minute longer. Cover bowl with plastic wrap and let dough rest 20 minutes.

knead the dough

Using fingers, create pocket in rested dough; add salt to pocket. Knead on low speed until dough is soft, smooth, and moist (dough should not be sticky), about 5 minutes. Transfer dough to clean work surface and knead by hand until dough forms firm ball, about 30 seconds.

let the dough rise

Lightly spray container or bowl with at least 4-quart capacity with nonstick cooking spray; place dough in container and lightly spray surface of dough. Take internal temperature of dough; then cover tightly with plastic wrap. If temperature registered below 78 degrees, set container at room temperature (about 70 degrees) in draft-free spot; if warmer than 78 degrees, set container at cool room temperature (about 65 degrees) in draft-free spot. Let stand until dough doubles in bulk, 3 to 5 hours.

stretch the dough

Scrape dough out onto clean work surface. Gently stretch dough (to redistribute and refresh yeast) as far as possible without tearing, then fold it into thirds like a letter.

divide and shape the dough

Using bench scraper or chef's knife, divide dough in half, each piece weighing about 1 1/2 pounds (680 grams). Form each half into rough ball, cover loosely with plastic wrap or damp kitchen towel, and let rest 15 minutes. To shape dough, use one hand to push dough against unfloured work surface, using other hand as guide. Goal is to make taught ball without ripping surface. Pinch bottom seam and set each round, seam side down, on separate sheets of parchment paper on dinner plates, rimless cookie sheets, or inverted rimmed baking sheets.

refrigerate the rounds overnight

Spray rounds lightly with nonstick cooking spray and cover loosely but completely with plastic wrap. Refrigerate overnight 8 to 12 hours.

proof and slash the rounds

Remove rounds from refrigerator and gently slide onto room-temperature surface where they can rise undisturbed for several hours; space them at least 6 inches apart. Loosen plastic wrap to allow rounds to rise; let rise until at least doubled in bulk and dough barely springs back when poked with your knuckle, 3 to 4 hours. Meanwhile, after about 2 hours, adjust oven rack to lower-middle position, place baking stone on rack, and heat oven to 500 degrees. Working one at a time, carefully slide rounds on parchment onto baking peel, rimless cookie sheet, or inverted rimmed baking sheet. Using sharp razor blade or knife held at 45-degree angle to work surface, slash surface of rounds 1/2 to 3/4 inch deep.

bake the bread

Working quickly, spray loaves with water, slide onto baking stone, and immediately reduce oven temperature to 450 degrees. During first 5 minutes of baking, spray loaves with water 2 additional times; bake until deep golden brown and instant-read thermometer inserted into center of loaves registers about 210 degrees, about 30 minutes total. Transfer loaves to wire rack, discard parchment, and cool loaves to room temperature on wire rack, about 2 hours.