In a small bowl stir together yeast and warm water and let stand 5 minutes, or until creamy. In a bowl stir together yeast mixture, room-temperature water, and flour and stir 4 minutes. Cover bowl with plastic wrap. Let sponge stand at cool room temperature at least 12 hours and up to 1 day.
In a small bowl stir together yeast and milk and let stand 5 minutes, or until creamy. In bowl of a standing electric mixer fitted with dough hook blend together milk mixture, sponge, water, oil, and flour at low speed until flour is just moistened and beat dough at medium speed 3 minutes. Add salt and beat 4 minutes more. Scrape dough into an oiled bowl and cover with plastic wrap. Let dough rise at room temperature until doubled in bulk, about 1 ½ hours. (Dough will be sticky and full of air bubbles.)
Have ready a rimless baking sheet and 2 well-floured 12- by 6-inch sheets parchment paper. Turn dough out onto a well-floured work surface and cut in half. Transfer each half to a parchment sheet and form into an irregular oval about 9 inches long. Dimple loaves with floured fingers and dust tops with flour. Cover loaves with a dampened kitchen towel. Let loaves rise at room temperature until almost doubled in bulk, 1 ½ to 2 hours.
At least 45 minutes before baking ciabatta, put a baking stone or 4 to 6 unglazed "quarry" tiles (see note, above) arranged close together on oven rack in lowest position in oven and preheat oven to 425° F. Transfer 1 loaf on its parchment to baking sheet with a long side of loaf parallel to far edge of baking sheet. Line up far edge of baking sheet with far edge of stone or tiles, and tilt baking sheet to slide loaf with parchment onto back half of stone or tiles. Transfer remaining loaf to front half of stone or tiles in a similar manner. Bake ciabatta loaves 20 minutes, or until pale golden. With a large spatula transfer loaves to a rack to cool.
Have a small bowl of water at your side when shaping the bread. This will make the sticky dough easier to handle.
Don't be alarmed if the bread loaves appear flat. They will rise and give you delicious results!
Use a stand mixer to make this recipe. If you mix by hand, you'll have the urge to add more flour in order to make the dough easier to work with. Adding too much flour will give you bread, but not ciabatta!