Parmesan Focaccia

This recipe comes from the test kitchen of Cook's Illustrated and the best I've ever tasted. So moist and light! The rapid rise yeast reduces the preparation time by more than an hour. If you use equal amounts of regular active dry yeast instead, let the sponge, in step 5 develop for 30 minutes rather than 20 minutes, and increase the first and second rises to one and one-half hours each.

• Ready In: 3 hrs 30 mins
• Serves: 6

Ingredients

• Dough

• 1  medium baking potato, peeled and quartered (about 9 ozs.)
• 1 1⁄2 teaspoons instant yeast
• 3 1⁄2 cups unbleached all-purpose flour
• 1  cup water (warm 105 to 115 degrees)
• 2  tablespoons extra virgin olive oil, plus more
•  extra virgin olive oil, for oiling bowl and pan
• 1 1⁄4 teaspoons table salt

• Topping

• 2  tablespoons extra virgin olive oil
• 2⁄3 cup  grated parmesan cheese (not shredded)

Directions

1. For the dough: Add potato to boiling water and simmer until done, about 25 minutes. Drain potato well and cool.
2. Once cooled put through fine disk on ricer or grate through large holes on grater box.
3. Reserve 1 1/3 cup lightly packed potato.
4. Meanwhile, in large bowl of electric mixer or work bowl of food processor fitted with steel blade, mix or pulse yeast, 1/2 cup flour, and 1/2 cup warm water until combined.
5. Cover tightly and set aside until bubbly, about 20 minutes.
6. Add remaining dough ingredients, including reserved potato. If using Kitchen Aid mixer, fit with paddle attachment and mix on low speed (number 2) until dough comes together.
7. Switch to dough hook attachment and increase speed to medium (number 4); continue kneading until dough is smooth and elastic, about 5 minutes.
8. If using food processor, process until dough is smooth and elastic, about 40 seconds. The dough will be wet. Transfer dough to lightly oiled bowl, turn to coat with oil, and cover tightly with plastic wrap. Let rise in warm, draft-free area until dough is puffy and doubled in volume, about 1 hour.
9. With wet hands (to prevent sticking), press dough flat into generously oiled 15 1/2 by 10 1/2 inch jelly roll pan, stretching it so that the dough reaches all sides of the pan and fills all four corners (If the dough resists going into the corners, cover it with a damp cloth and let it relax for 15 minutes.) Once the dough has been stretched into the pan cover with a lightly oil-sprayed plastic wrap; let rise in warm, draft-free area until dough is puffy and double in volume, 45 minutes to 1 hour.
10. Meanwhile, adjust oven rack to lower-middle position and heat to 425°F.
11. Once doubled in size, with two wet fingers, dimple risen dough.
12. Drizzle the dough with the topping oil and sprinkle evenly with Parmesan cheese.
13. Bake until focaccia bottom is golden brown and crisp, 23 to 25 minutes. Transfer to wire rack to cool slightly. Cut into squares and serve with olive oil and balsamic vinegar.