Kitchen scalenote that I recommend measuring the ingredients for this recipe by weight (tap on "metric" in ingredients if the weight measures are not displayed)
Whisk together 275 grams all-purpose flour, 100 grams whole wheat flour, 6 grams instant yeast, 9 grams table salt, and 6 grams granulated sugar in the bowl of a stand mixer. Stir in 284 grams warm water until a shaggy dough is formed.
Fit the stand mixer with a dough hook, and turn the mixer on low speed. Mix until all of the flour is mixed in, and a very sticky dough is formed (you may need to turn off the mixer and scrape down the sides of the bowl several times). Note that the dough will continue to stick to the bottom, and even up the sides of the bowl.
Lightly coat the insides of a medium to large bowl with 1 Tablespoon of olive oil. Use a spatula to scrape the dough into the bowl. Turn the dough over in the bowl so that it's mostly coated with some of the olive oil. Cover the bowl with a lid, a plate, a damp kitchen towel, or some plastic wrap and set it aside to rise for 1 hour.
After an hour, coat the inside of of an 8x8" baking pan with 2 Tablespoons of olive oil, turning the pan to ensure that the entire bottom and the sides of the pan are coated with oil.
Turn the dough into the prepared pan, turning the dough over to further coat it with the oil. Gently press the dough into the pan, attempting to stretch it into the corners (don't worry if the dough doesn't stay put). Cover the pan with a damp kitchen towel or plastic wrap, and set it aside to rise for 45 more minutes. After 30 minutes, move the oven rack to the lower-middle position and heat the oven to 425°F.
Remove pan cover and drizzle the top of the dough with an additional 1 Tablespoon of olive oil. Using oil-coated fingertips, gently press your fingers into the dough, all over the top of the dough to create dimples. If desired, sprinkle the top of the dough with chopped rosemary, up to 1/4 teaspoon kosher salt, or another topping.
Bake the bread until the top is golden brown, about 20-25 minutes. Run a paring knife between the bread and the pan, and if the bottom is sticking, use a spatula or flipper to carefully pry the bread out. For crispier results, place the loaf of focaccia back into the oven once it's removed from the pan, directly on the oven rack. Cook for about 5 minutes. Cut into squares or rectangles to serve. If desired, serve with olive oil and balsamic vinegar for dipping.
Notes
If you only have regular (active dry) yeast, you can feel free to use that instead. Use the same amount, but instead of mixing the yeast directly into the flour, follow these steps:
Stir the granulated sugar into the warm water until dissolved.
Sprinkle the yeast on top of the water mixture and let it sit for about 10 minutes until the yeast becomes slightly puffy.
Stir the yeast into the water. Stir the water mixture into the flour mixture and follow the recipe as written.