Mix in butter. To a large bowl, add 4 cups All-purpose flour and ½ cup Cold salted butter (grated). Using a fork or your hands, stir well. You are looking for a crumbly texture (the butter should be distributed throughout the flour).

Mix in yeast, salt and water. Add 2 tsp Instant yeast, 1 tsp Kosher salt and 2 cups Cold water. Mix with a wooden spoon until all the flour is hydrated. The mix should be shaggy and there should be no dry flour streaks.

Rise. Drizzle 1-2 tsp Extra virgin olive oil over the top of the dough, cover the bowl with plastic wrap or a tight-fitting cover and refrigerate overnight for at least 18 hours.

Deflate and shape. 2 hours before you want to bake the focaccia, take the bowl out of the fridge and uncover it. Using your hands or a dough scraper deflate the dough slightly and transfer the dough to a generously buttered and oiled baking tray (9x13) or sheet pan. Shape it loosely by grabbing each end and folding it over like you would a letter. Flip the dough over so the seams are on the bottom. Cover the tray with a towel and set aside for 1.5-2 hours.

Dimple. Preheat your oven to 450F/230C. The dough should now have spread covering the surface of the baking tray. If not, you can help it stretch to the edges by carefully easing it to the corners. Drizzle 2 tbsp Butter (Melted) over the dough and start dimpling with your fingers, covering the entire surface. This creates valleys and mountains in the dough and you should also see air bubbles forming. Add a big pinch of Flaky salt.

Bake. Bake in the preheated oven for 20-22 minutes, until golden brown. Every oven is different, so your focaccia may need more time. Let the focaccia cool for 30 minutes before cutting into it.