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

In a large mixing bowl, whisk together bread flour and salt. Pour in the activated yeast mixture along with olive oil. Stir until a sticky dough forms, then knead gently for 3-4 minutes.

Shape the dough into a ball and place it in a greased bowl. Cover with plastic wrap and refrigerate for at least 12 hours.

Grease a 9x13-inch baking pan with melted unsalted butter. Deflate the chilled dough and gently stretch it into the pan. Cover with a clean kitchen towel and let it rise in a warm spot for 1.5 to 2 hours.

Melt unsalted butter over medium heat until golden brown, stirring constantly. Allow it to cool slightly.

Mix light brown sugar and cinnamon in a small bowl. Combine with the cooled brown butter.

Preheat oven to 450°F (230°C). Create dimples all over the surface of the dough in the baking pan. Drizzle the cinnamon-brown butter mixture over the dimples.

Bake for 20-25 minutes until golden brown. Let cool in the pan for 10 minutes.

Mix powdered sugar, milk, and vanilla extract in a bowl until smooth. Drizzle over the warm focaccia before slicing.