1 ¾cups plus 2 tablespoons(15 ounces) warm water (85 to 90°F)
½teaspoonactive dry yeast
1 ½cups(12 ounces) biga starter(recipe follows)
4 ½cups(22.5 ounces) unbleached bread flour
2tablespoons plus 2 teaspoons(1.48 ounces) milk
2tablespoons plus 2 teaspoons(1.27 ounces) extra-virgin olive oil
1tablespoon plus 1 ¼ teaspoons(0.45 ounces) kosher salt
2tablespoons plus 1 teaspoon(0.35 ounces) fresh rosemary, about 2 ½ branches, chopped
Additional extra-virgin olive oil and kosher saltfor topping
Line a 17 by 12-inch sheet pan with parchment paper and lightly oil with olive oil. Set aside.
Place the warm water and yeast in a large bowl. Stir with a fork to dissolve the yeast and allow to stand for about 3 minutes. If you are working in a cool kitchen on a cool day, increase the water temperature to 105°F to give the dough a warmer start.
Add the biga to the yeast mixture and mix with your fingers for 1 to 2 minutes to break it up. The mixture should look milky and foamy. Add the flour and mix in with your hands, lifting the wet mixture over the flour to incorporate it. When the dough becomes a shaggy mass, move to a very lightly floured surface and knead until it becomes smooth and somewhat elastic, about 5 minutes. Place the dough back into the mixing bowl, cover with oiled plastic, and let rest for 20 minutes to smooth out and develop elasticity.
After the rest period add the milk, oil, and salt to the dough in the mixing bowl and knead it in the bowl until it is all incorporated.
Move the dough to a lightly floured work surface and knead until it is very smooth, silky, and elastic, 7 to 10 minutes. The dough will be sticky, but don’t use too much flour for kneading. The finished dough should be wet but supple and springy.
Put the dough in a lightly oiled bowl, turn it to coat with oil, and cover it tightly with oiled plastic wrap. Let the dough rise at room temperature (75 to 77°F) for 1 hour.
Turn the dough while it is still in the mixing bowl. Gently deflate the dough in the middle of the bowl with your fingertips, then fold the left side over the middle, and the right side over the middle. Fold the dough in half, gently pat it down, and then turn it over so the seam is underneath. Let it rise again for 1 to 1 ½ hours, until nearly doubled in volume.
When the dough has risen, loosen it from the bowl and gently pour it onto the center of the oiled baking sheet. Pat it gently with your fingertips to stretch it evenly out to the edges of the pan. Be careful not to tear the dough. If the dough resists stretching, let it rest for 2 to 5 minutes, until it becomes supple enough to stretch again, then continue to press it out to the edges of the pan. (If the dough is dry, you may have to repeat the resting/stretching procedure several times). Brush the top of the dough lightly with olive oil, cover with lightly oiled plastic wrap, and let rise for 1 to 2 hours, until the dough has doubled and fills the pan (a finger pressed into the dough will leave an indentation).
Thirty minutes before baking, preheat the oven to 425°F. Fill a plastic spray bottle with water and place a small pan (such as a mini loaf pan) on the lowest possible rack of the electric oven. If using a water pan to create steam, also place a cast-iron pan (that you are willing to get rusty) next to the small pan, fill a teakettle with water to be boiled later, and have a metal 1-cup measure with a straight handle available near the kettle.
Five to 10 minutes before the focaccia is ready to bake, carefully place 2 or 3 ice cubes in the small pan in the bottom of the oven. This helps to create moisture in the oven prior to baking. If using a water pan, also turn the water on to boil.
Brush and dot the surface of the dough gently with olive oil, dimple it in several spots with your fingertips to prevent air pockets from developing underneath, and sprinkle the surface lightly with kosher salt. Sprinkle with chopped rosemary all the way to the edges.
Quickly open the oven, and place the pan of focaccia on the oven rack, then using the plastic spray bottle, quickly mist the focaccia 6 to 8 times. If using a water pan, have the metal 1-cup measure already filled with boiling water and carefully pour it into the cast-iron skillet. If not using a water pan, instead quickly spray the walls of the oven 8 to 10 times. Immediately close the oven door. Two minutes later, open the oven and quickly spray the walls 8 to 10 times more, closing it immediately afterward. Spray the walls again 2 minutes later, 8 to 10 times (you will have sprayed the oven walls on 3 separate occasions now if not using a water pan).
Bake for 15 minutes, then reduce the oven temperature to 350°F and bake for 15 to 20 minutes longer, until golden brown and crusty but still soft inside.
Remove the focaccia from the oven and immediately brush it lightly with olive oil. Cool in the pan 10 minutes, then carefully slide it onto a cooling rack. Remove the parchment (to prevent steam from softening the bottom crust) and let cool. Serve warm or at room temperature, cut into squares. Focaccia is best served the day it is baked.