Combine warm water and sugar in a bowl. Sprinkle yeast and let sit for 10-15 minutes until frothy.

Whisk bread flour and salt in a large bowl. Add yeast mixture and olive oil, stirring until a sticky dough forms.

Transfer dough to a greased bowl, cover, and let rise in the refrigerator for at least 12 hours.

Preheat oven to 400°F. Grease a baking pan, deflate the dough, and transfer to the pan, letting it rise for 1.5 to 2 hours.

Melt unsalted butter in a saucepan. Add light brown sugar and cinnamon, mixing until dissolved.

Dimple risen dough with fingers. Drizzle cinnamon mixture over it.

Bake for 20-25 minutes until golden brown and set.

Mix powdered sugar, milk, and vanilla to create icing, and drizzle over the warmed focaccia.