Lightly spoon 4 cups flour into dry measuring cups; level with a knife. Combine with sponge and next 4 ingredients (sponge through salt) in a food processor; process until dough forms a ball. Process 1 additional minute.
Turn dough out onto a floured surface (dough will be sticky and soft); divide dough in half. Working with one portion of dough at a time (cover the remaining dough to keep from drying), roll each portion into a 12 x 6-inch rectangle. Place dough on a parchment paper-lined baking sheet sprinkled with cornmeal. Taper ends of dough to form a "slipper" shape. Sprinkle 2 tablespoons flour over loaves. Cover and let rise 30 minutes or until doubled in bulk.
Preheat oven to 425°.
Uncover dough. Bake at 425° for 25 minutes or until loaves sound hollow when tapped. Remove from pan; cool on a wire rack.