500 grams bread or all-purpose flour (17.5 ounces; about 3 1/4 cups), see note
10 grams Diamond Crystal kosher salt (0.4 ounce; 2 1/2 teaspoons); for table salt, use about half as much by volume or the same weight
4 grams instant dry yeast, such as SAF (0.1 ounce; 1/2 packet or 1 rounded teaspoon)
400 grams room temperature water (14.1 ounces; 1 1/2 cups plus 3 tablespoons)
68 grams extra-virgin olive oil (2.4 ounces; 5 tablespoons), divided
Coarse sea salt, such as Maldon or fleur de sel
For the Dough: Combine flour, salt, and yeast in large bowl and whisk together until homogenous. Add water and stir with wooden spoon until no dry flour remains, about 2 minutes. Add 20 grams (0.7 ounces; 1 1/2 tablespoons) olive oil and stir to incorporate, using hands if necessary to work oil into dough. Cover with plastic wrap, then let rest at room temperature for 1 hour.
Remove plastic wrap (but don’t discard), and, using a lightly greased bowl scraper, fold dough over itself by lifting and folding edge of dough toward middle. Turn bowl 45 degrees and fold dough again; repeat turning and folding motion 4 more times (total of 6 folds). Cover tightly with reserved plastic wrap and let rest for 15 minutes.
Repeat this entire folding process once more. Cover bowl tightly with plastic wrap, and refrigerate for at least 18 hours or up to 3 days.
Baking the Focaccia: Remove dough from refrigerator and let sit at room temperature for 10 minutes. Sprinkle the top of the dough lightly with flour, then transfer it to a lightly-floured work surface. Shape it by holding it with well-floured hands and tucking the dough underneath itself, rotating it until it forms a tight ball.
Pour 27 grams (1 ounce; 2 tablespoons) olive oil into 10- or 12-inch cast iron skillet and spread over entire inner surface (including rim) with your hands. Transfer dough to pan, turn to coat in oil, position seam-side-down, and spread gently with your hands to mostly fill the pan (don’t worry if dough doesn’t fully stretch to edges). Cover tightly with plastic wrap and let rise at room temperature until dough is very soft and puffy and nearly doubled in volume, 1 1/2 to 2 hours.
One hour before baking, adjust oven rack to lower-middle position. Place baking stone or Baking Steel on it, and preheat oven to 500°F (260°C). Using your fingertips, gently press and stretch dough to evenly fill all corners of the pan. Lift up one edge of the dough to let air bubbles underneath escape and repeat, moving around the dough until there are no large air bubbles left underneath and the dough is evenly spread around the pan. Using fingertips, gently press on surface of dough to create small dimples. Drizzle remaining 20 grams (0.7 ounces; 1 1/2 tablespoons) olive oil over top of dough, then sprinkle with coarse sea salt.
Transfer skillet to oven, positioning it on top of baking stone, and bake, rotating pan halfway through baking, until top is golden brown and bottom is golden brown and crisp when you lift it with a thin spatula, 25 to 30 minutes. Let focaccia cool in skillet for 5 minutes, then use a thin spatula to transfer focaccia to wire rack set inside a rimmed baking sheet. Spoon or brush any olive oil left in the skillet over the focaccia, and let cool for at least 10 minutes before slicing. Serve warm or at room temperature.