If making the bread in a stand mixer: Stir the 1 teaspoon active dry yeast into the 5 tablespoons (2 1/2 ounces) warm milk in a mixer bowl; let stand until creamy, about 10 minutes. Add the 1 cup plus 3 tablespoons (9 1/2 ounces) water, 1 tablespoon (1/2 ounce) olive oil, and 2 very full cups (17 1/2 ounces) biga (be sure to weigh the biga, don't just measure it by volume) and mix with the paddle until blended. Mix the 3 3/4 cups (17 1/2 ounces) unbleached all-purpose flour (be sure to weigh the flour, don't just measure it by volume) and 1 tablespoon (1/2 ounce) salt, add to the bowl, and mix for 2 to 3 minutes. Change to the dough hook and knead for 2 minutes at low speed, then 2 minutes at medium speed. The dough will be very sticky. Knead briefly on a well-floured surface, adding as little flour as possible, until the dough is still sticky but beginning to show evidence of being velvety, supple, springy, and moist. (If the dough seems almost impossibly sticky to work with, reread the headnote above from author Carol Field.) Place the dough in an oiled bowl, cover with plastic wrap, and let rise at room temperature until doubled, about 1 1/4 hours. The dough should be full of air bubbles, supple, elastic, and sticky.
Turn the dough onto a generously floured surface and cut it into 4 equal portions. Roll each portion into a cylinder, then stretch each cylinder into a rectangle about 10 by 4 inches, pulling with your fingers to get each portion of dough long and wide enough.
Generously flour 4 pieces of parchment paper placed on peels or upside-down baking sheets. Place each loaf, seam side up, on a piece of parchment. Dimple the loaves vigorously with your fingertips or knuckles so that they won’t rise too much. The dough will look heavily pockmarked, but it is very resilient, so don’t be concerned.
Cover the loaves loosely with damp towels and let rise until puffy but not doubled in size, 1 1/2 to 2 hours. The loaves will look flat and definitely unpromising, but rest assured that they will rise more in the oven.
About 30 minutes before baking, preheat the oven to 425°F (220°C) and slide 2 baking stones on the center rack to heat. [Editor's Note: If, like us, you haven’t yet bought yourself a baking stone, flip some large cast-iron skillets upside down and bake the bread on their bottoms. It ought to do the trick. It has for us.]
Just before baking the bread, sprinkle the stones with cornmeal. Carefully invert each loaf onto a stone. If the dough sticks a bit to the parchment, just gently work it free from the paper. If you need to, you can leave the paper on and remove it 10 minutes into baking.
Bake the ciabatta for a total of 20 to 25 minutes, spraying the oven 3 times with water in the first 10 minutes. The loaves are done when an instant-read thermometer reads 205°F (96°C) to 210°F (99°C). Transfer the loaves to wire racks to cool.