Make the dough: In a large bowl, whisk together the flour, salt, and instant yeast. Add the water. Using a rubber spatula, mix until the liquid is absorbed and the ingredients form a sticky dough ball. Cover the bowl and let it rest for 30 minutes.

Fill a small bowl with water. Using a wet hand, stretch and fold the dough by grabbing an edge and pulling it up and towards the center. Repeat this stretching and folding process, 8 to 10 times, moving your hand around the edge of the dough with every set of stretches and folds. As you stretch and fold, you should feel the dough transform from being sticky and shaggy to smooth and cohesive. Find video guidance in the post above as well as here.

Rub the surface of the dough lightly with olive oil. Cover the bowl with a lid or plastic wrap and place in the refrigerator immediately for at least 12 hours or for as long as three days. (See notes above if you need to skip the overnight rise for time purposes.)

Grease a 9×13-inch ceramic pan with softened butter. (Note: This greasing step may seem excessive, but with some pans, it is imperative to do so to prevent sticking. With my USA pans, I can get away with olive oil alone; with my glass baking dishes, butter is a must.) Pour 2 tablespoons of oil into the pan.

Using a flexible bench scraper or a lightly oiled hand, deflate the dough by releasing it from the sides of the bowl and pulling it toward the center. Rotate the bowl in quarter turns as you deflate, turning the mass into a rough ball. Turn the dough out into the olive oil in the prepared pan. Turn the dough several times in the oil to ensure it is completely coated. Let the dough rest for 1.5 hours. Cover the pan. (I use a cutting board or sheet pan.)

In a small skillet, melt 4 tablespoons of the butter. Whisk in 4 tablespoons of the brown sugar, 2 teaspoons of the cinnamon, and a pinch of flaky sea salt. Remove from the heat and let cool briefly.

Using lightly oiled hands, gently dimple the dough, stretching it to fit the pan. Pour the brown sugar filling as evenly over the top of the dough as possible, and use your hands to distribute it over the surface. Reserve the skillet.

Starting with a short end, fold the dough envelope style: if, for example, you are starting with the right edge, fold it to the left covering two-thirds of the dough, then fold the left edge over to the right to cover. Then turn the bundle of dough so that the open ends face the short ends of the pan. Cover the pan and let the dough rest for another 1.5 hours.

At this point you can cover the dough with saran wrap and refrigerate overnight. Take out of frig and allow to come to room temperature 1 hour before baking.

In the same small skillet, melt the remaining 4 tablespoons of butter. Whisk in the remaining 4 tablespoons of brown sugar, remaining 2 teaspoons of cinnamon, and a pinch of flaky sea salt. Remove from the heat and let cool briefly.

Uncover the pan, pour the filling over the surface of the dough, and rub with your hands to distribute it evenly. Using lightly oiled hands, gently dimple the dough, creating bubbles and craters as you do. Sprinkle lightly with sea salt.

Transfer the pan to the oven and bake for 25 minutes or until the focaccia is evenly browned. Remove the pan from the oven, let the focaccia cool in the pan briefly (2 to 3 minutes), then run a spatula around the edges of the pan, and transfer the focaccia to a cooling rack to cool for 5 minutes.

In a small bowl or liquid measure, whisk together the confectioners’ sugar, milk, and vanilla.

Transfer the focaccia to a serving board. Drizzle the glaze over the top. Using a serrated knife, cut the focaccia into pieces (or more) and serve.