Add the bread flour, instant yeast, and kosher salt to a large mixing bowl and whisk very well to evenly distribute the yeast and salt.
Add the warm water and 2 tablespoons of olive oil to the dry ingredients.
Mix very well until everything is completely combined and you have a uniform, loose dough with no pockets of dry flour. Cover the bowl and place it in a warm spot in your kitchen for 15 minutes.
Uncover the dough and perform the first set of folds. Using a moistened hand, a bowl scraper, or two forks, grab the dough at one edge, pull it up, and fold it over to the opposite side.
Turn the bowl 90 degrees and repeat until you have gone all the way around the bowl for a total of four folds. Cover the bowl and let the dough rest for another 15 minutes.
Repeat the same set of four folds. Cover the bowl again and let the dough rest for a final 15 minutes.
Add the remaining 2 tablespoons of olive oil to a 9 x 9-inch baking pan and scrape the dough into the pan. Drizzle the top with a little more olive oil and use your fingertips to gently coax the dough toward the corners.
It does not need to reach all the way. Cover the pan with plastic wrap and refrigerate overnight, up to 12 hours.
In the morning, preheat the oven to 450 degrees F. Remove the focaccia dough from the refrigerator, uncover it, and let it sit at room temperature for about 45 minutes.
Drizzle the dough with a little more olive oil. Press your fingertips deeply into the dough, almost to the bottom of the pan, creating dimples across the surface.
Sprinkle with flaky sea salt and bake for 25 to 30 minutes, or until deeply golden brown. Let the focaccia rest in the pan for 5 minutes, then transfer it to a wire rack to cool completely.
OPTIONAL ROSEMARY GARLIC FOCACCIA
Combine the olive oil, chopped rosemary, grated garlic, and kosher salt. After the dough has rested at room temperature, drizzle the rosemary garlic oil over the top instead of using plain olive oil.
Use your fingertips to dimple the dough deeply, pressing some of the rosemary and garlic into the dimples as you go. Bake as directed.