Foolproof Focaccia - cooking recipe

Ingredients

Starter
1 1/2 teaspoons fast rising yeast
1/2 cup water
1/2 cup flour
Dough
1 medium baking potato, peeled and quartered (about 9 ounces)
3 cups unbleached all-purpose flour
1/2 cup warm water, 105-115 degrees
2 tablespoons extra virgin olive oil
1 1/4 teaspoons salt
TOPPING
2 tablespoons extra virgin olive oil
2 tablespoons fresh rosemary leaves (I usually omit)
3/4 teaspoon coarse sea salt (or 1-1/4 teaspoons kosher salt)

Preparation

FOR THE DOUGH:
Boil potato in water and simmer until tender (about 25 minutes).
Drain potato well and cool.
Grate potato through large holes on box grater, fine disk on ricer or food processor.
Reserve 1-1/3 cups lightly packed potato.
Meanwhile, In food processor mix or pulse yeast, 1/2 cup flour, and 1/2 cup warm water until combined.
Cover tightly with plastic wrap and set aside in a warm place, until bubbly (about 20 minutes).
Add remaining dough ingredients (including the remaining 3 cups flour), including reserved potato.
Process dough for about 40 seconds until it's smooth and elastic.
Transfer dough to lightly oiled bowl, turn to coat with oil and cover tightly with plastic wrap.
let rise in warm area until dough is puffy and doubled in volume (about an hour).
With wet hands (to prevent sticking) press dough flat into oiled or sprayed with Pam 15 1/2 by 10 1/2 jelly roll pan (I sprinkle a little flour on top of dough and gently pat it into shape. I don't make it quite as large, since I like it really thick).
Cover dough with lightly greased plastic wrap (dough will stick otherwise) and let rise in warm area, about 45 minutes to an hour, until dough is puffy and doubled.
Preheat oven to 425\u00b0F.
Adjust oven rack to lower middle position.
With two wet fingers, dimple risen dough at regular intervals.
TOPPING:
Drizzle dough with oil, and sprinkle evenly with the rosemary, salt, landing some in pools of oil.
Bake until focaccia bottom is golden and crisp, 23-25 minutes.
Yields: one 15 1/2 by 10 1/2 inch rectangle or 2 8 inch rounds.
*Note: focaccia can be kept on counter for several hours and reheated just before serving.
Or, wrap cooled focaccia in plastic wrap, and then foil and freeze for up to 1 month (unwrap and defrost in 325\u00b0F oven until soft, about 15 minutes).
VARIATIONS:
Parmesan Focaccia: substitute 2/3 cup grated Parmesan cheese for the rosemary and coarse sea salt.
Focaccia with Black Olives and Thyme: substitute 1 teaspoon fresh thyme leave and 24 pitted large black olives (one per oil-filled dimple) for rosemary.