In a medium glass bowl, stir together the yeast, lukewarm water and sugar. Let the mixture stand until foamy, about 5 minutes. Stir in 2 tablespoons of the olive oil and 1 1/2 cups of the flour. Cover the bowl with plastic wrap and set in a warm spot for about 2 hours, until the mixture has doubled in volume.
Remove the plastic wrap and stir in the cool water, 1 1/2 teaspoons of salt, the whole wheat flour, the remaining 1 cup of all-purpose flour and the chopped rosemary to form a wet dough. Transfer the dough to a mixer fitted with a paddle attachment and beat for 3 to 5 minutes on medium-low speed.
Lightly coat the inside of another medium bowl with oil. Transfer the dough to the bowl, cover with plastic wrap and refrigerate for 12 to 24 hours. Remove the dough from the refrigerator 2 hours before baking to give it time to warm up. The dough will be bubbly and loose.
Preheat the oven to 400 degrees. Position a rack on the middle shelf. Brush a cast-iron skillet (or 2-quart baking dish) with 1 tablespoon of olive oil.
Gently scrape the dough into the skillet. Drizzle with the remaining tablespoon of olive oil. Sprinkle with salt and top with rosemary leaves.
Set the dough aside to rise for 20 minutes, then bake until golden brown, about 40 minutes. Cool the focaccia in the pan for at least 20 minutes before slicing.