In a large mixing bowl, combine room temperature water, instant yeast, bread flour, and fine sea salt until no dry flour remains.

Let the dough rest for 10 minutes before adding melted unsalted butter and gently folding it in until fully incorporated.

Transfer the dough to a lightly oiled bowl, cover with a damp cloth, and let it rise at room temperature for about 2 hours.

Prepare a 9x13-inch baking pan by coating it with olive oil and melted unsalted butter.

Once the dough has risen, stretch it gently to fill the pan and spread half of the cinnamon-sugar filling over one half.

Fold the dough over itself and let it rise again for 1 to 2 hours, covered with a damp cloth.

Preheat your oven to 400°F (200°C).

Drizzle the remaining filling across the dough surface and dimple the dough with oiled fingertips.

Bake for 18-22 minutes, until golden-brown and reaching an internal temperature of 190°F (88°C).

Allow to cool in the pan for about 20 minutes before whisking together the icing ingredients and drizzling over the cooled focaccia.