In a small bowl, combine warm water with granulated sugar. Sprinkle active dry yeast, stir gently, and let sit for 10-15 minutes until bubbly.

In a large mixing bowl, whisk together bread flour and salt. Pour in yeast mixture and olive oil; stir until a sticky dough forms. Oil the surface and cover with plastic wrap.

Refrigerate the dough for at least 12 hours for the first rise.

Grease a baking pan with melted unsalted butter. Turn the dough onto a floured surface, deflate slightly, and transfer to the pan. Stretch to fill the pan, cover with a towel, and let it rise in a warm place for 1 ½ to 2 hours.

Prepare the brown butter in a saucepan over medium heat, melting unsalted butter until golden brown and nutty (about 5-7 minutes). Let cool.

In a small bowl, mix light brown sugar with cinnamon and add to cooled brown butter. Stir until combined.

Preheat the oven to 450°F. Dimple the dough and drizzle with the cinnamon-brown butter mixture.

Bake for 20-25 minutes until golden brown. Let cool for about 10 minutes.

Whisk together powdered sugar, milk, and vanilla to make icing. Drizzle over the warm focaccia.