In a large mixing bowl, combine the bread flour, lukewarm water, active sourdough starter, sugar, and salt. Mix until you have a shaggy dough. Cover with a clean kitchen towel and let rest for 30 minutes — this is the autolyse stage, and it's doing serious gluten-building work.

After the autolyse rest, add the softened butter and work it into the dough using stretch-and-fold motions until fully incorporated. The dough will feel slippery at first — just keep going, it comes together!

Transfer the dough to a lightly oiled bowl and cover. Let it ferment at room temperature for 5–10 hours, or overnight.

During the first 2 hours, perform stretch and folds every 30 minutes: grab one side of the dough, stretch it up, fold it over the center, rotate the bowl, and repeat 4 times per set. This builds the structure that creates those gorgeous airy bubbles.

In a small bowl, mix together the packed brown sugar, ground cinnamon, and melted butter until you have a paste-like mixture. Set aside until ready to use.

Once the dough has fermented and is puffy and full of bubbles, transfer it to a well-greased 9x13 pan (or two 8x8 pans). Gently stretch to fill — if it springs back, rest for 10 minutes and try again.

Use your fingers to poke deep dimples all over the surface of the dough. Drizzle 2 tablespoons of melted butter over the top, then sprinkle generously with the cinnamon roll topping, pressing it lightly into the dimples.

Bake at 375°F (190°C) for 25–30 minutes until deep golden brown and the topping is bubbling and caramelized.

While the focaccia bakes, whisk together the powdered sugar, milk or cream, vanilla extract, and cinnamon until smooth.

The moment the focaccia comes out of the oven, drizzle the icing generously all over the top. It will melt into the warm bread and set as it cools. Let cool at least 10 minutes before slicing and serving.