In the bowl of a stand mixer with the dough hook attachment, or by hand, mix together all of the focaccia dough ingredients.At first, the dough will tear when stretched.
After about 10 minutes of mixing, the dough will finally be strong enough to pass the windowpane test.Stretch the dough between your fingers and see if light can pass through.
Cover the dough and place in the refrigerator to continue proofing overnight.At the end of proofing, the dough will double in size and be full of bubbles.
Dump the cold dough into a 9x13" baking pan, spread out to the corners, and proof again for about 4 hours.The dough will double in size and be very airy.