Activate the yeast. In a small bowl, whisk warm water with granulated sugar. Sprinkle in the yeast and let stand 10–15 minutes until foamy.

Mix the dough. In a large bowl, whisk together flour and salt. Add the foamy yeast mixture and olive oil, stirring until a sticky dough forms. Lightly oil the dough surface.

First rise. Cover the bowl and refrigerate for at least 12 hours (overnight).

Second rise. Grease a 9×13-inch pan (line with parchment), drizzle with 2 tablespoons melted butter. Deflate the dough gently and fold it into the pan. Cover and let rise 1½–2 hours in a warm spot.

Prepare topping. Melt and brown ½ cup butter (see note). Once cooled, whisk in brown sugar and cinnamon until dissolved.

Dimple and top. Preheat oven to 450 °F. Wet your fingers and press deep dimples all over the risen dough. Drizzle the cinnamon-butter topping evenly across the surface.

Bake. Bake 20–25 minutes until golden and caramelized. Cool in the pan for about 10 minutes.

Finish with icing. Stir together powdered sugar, vanilla, and milk until smooth. Drizzle over the warm focaccia. Slice into squares and serve.