Mix the dough: In a large bowl, whisk 120 g active starter with 375 g water until milky. Add 500 g bread flour and 10 g fine sea salt. Stir until no dry spots remain. The dough will be shaggy and sticky.

Rest and strengthen: Cover and rest 30 minutes. Perform 3 rounds of stretch-and-folds every 20–30 minutes to build structure. Lightly oil your hands if needed. The dough should become smoother and slightly bouncy.

Bulk rise: Cover and let rise at room temp until puffy and about 50% larger, 3–5 hours depending on warmth. For more flavor and flexibility, transfer to the fridge and cold-proof 8–18 hours.

Prep the pan: Generously oil a 9x13-inch pan or a half sheet pan with 2–3 tablespoons olive oil. This helps crisp the bottom crust.

Pan the dough: Gently coax the dough into the pan. If it springs back, rest 10 minutes, then stretch again until it mostly fills the pan.

Final proof: Cover and let the dough relax and puff until airy and slightly jiggly, 45–90 minutes. Warm kitchens move faster.

Make the cinnamon butter: Melt 60 g unsalted butter. Stir in 100 g granulated sugar, 2–3 teaspoons ground cinnamon, and 1 teaspoon vanilla. It should be syrupy and spoonable.

Dimple and swirl: With oiled fingers, press deep dimples across the dough. Spoon the cinnamon mixture over the top, letting it pool in the dimples. Use a spoon to drag gentle swirls so it spreads but doesn’t flood the edges.

Bake: Heat oven to 425°F (220°C). Bake 18–25 minutes, until the top is golden and the edges are sizzling. If browning too fast, tent loosely with foil for the last few minutes.

Glaze: Whisk 120 g powdered sugar with 1–2 tablespoons milk and a splash of vanilla until smooth and pourable. While the focaccia is warm but not hot, drizzle in zigzags. Optional: finish with a pinch of flaky sea salt.

Cool and slice: Let rest 15–20 minutes. Lift from the pan, then cut into squares. Serve warm or at room temp.