Directions:

STEP 1. In the bowl of a stand mixer fitted with the paddle attachment, whisk together ¾ cup (180 grams) warm water and instant sourdough by hand until dissolved. Add 1⅓ cups (169 grams) flour, and beat at low speed until combined, about 30 seconds. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size, 30 to 45 minutes. Add salt, remaining 2⅔ cups (339 grams) flour, and remaining ½ cup (120 grams) warm water, and beat at low speed until dough comes together, about 30 seconds. Switch to the dough hook attachment. Beat at low speed for 2 minutes.
STEP 2. Lightly oil a large bowl. Place dough in bowl, turning to grease top. Cover and let stand in a warm, draft-free place (75°F/24°C) until dough is smooth and elastic, about 1½ hours, folding every 30 minutes.
STEP 3. Turn out dough onto a lightly floured surface, and divide in half (about 296 grams each). Shape each half into a blunt log by gently flattening dough into a rectangle. Take one edge of short side of dough, and tuck over center third, pinching to seal. Fold remaining third over center, pinching to seal. Roll dough until seam side down. Cover and let rest for 15 minutes.
STEP 4. Place a baguette pan on a rimmed baking sheet. Line each well of baguette pan with thin strips of parchment paper. Dust with bread flour.
STEP 5. Shape baguettes by gently flattening each into an 8x6-inch rectangle so one long side is nearest you. Starting with top edge, fold top third of dough to center, pressing to seal. Fold bottom third over folded portion, pressing to seal. Fold dough in half lengthwise so long edges meet. Using the heel of your hand, firmly press edges to seal. Roll into a 16-inch log, tapering ends slightly. Place logs, seam side down, on prepared pan. Dust with bread flour. Cover and refrigerate 8 hours or overnight.
STEP 6. Remove dough logs from refrigerator, and let rise in a warm, draft-free place (75°F/24°C) until slightly puffed, about 45 minutes.
STEP 7. Preheat oven to 475°F (250°C).
STEP 8. Using a lame or razor blade, score baguettes with long diagonal cuts. Place ice cubes on rimmed backing sheet. Using a large sheet of heavy-duty foil, loosely cover loaves (do not allow foil to touch loaves), sealing edges tightly around rim of baking sheet.
STEP 9. Bake for 15 minutes. Remove foil and bake until golden brown and an instant-read thermometer inserted in center registers 205°F (96°C), 5 to 10 minutes more. Let cool on a wire rack.