Combine warm water and granulated sugar in a bowl. Sprinkle active dry yeast over the mixture and let it sit until foamy, about 10-15 minutes.

In a separate bowl, whisk together bread flour and salt. Pour in the yeast mixture and olive oil, stirring until a sticky dough begins to form. Lightly oil the surface and shape the dough into a ball.

Cover the bowl with a towel or plastic wrap, then refrigerate for at least 12 hours.

Once the dough has risen, grease a 9x13-inch pan with melted unsalted butter. Gently deflate the dough and transfer it to the pan, spreading it into an even layer.

Cover the dough and place it in a warm location to rise again for 1.5 to 2 hours, or until nearly doubled in size.

In a small saucepan, melt unsalted butter over medium heat until golden brown. Combine with light brown sugar and cinnamon.

Preheat your oven to 450°F (230°C). Press dimples into the risen dough and drizzle the brown butter mixture over the top. Bake for 20-25 minutes until golden brown.

Whisk together powdered sugar, milk, and vanilla extract until smooth. Drizzle over the warm focaccia and slice into generous squares.