Mix the dough: In a large bowl, combine 100 g active starter, 375 g water, and 450 g bread flour. Stir to a shaggy mass. Rest 20–30 minutes (autolyse).

Add salt and oil: Sprinkle in 10 g fine sea salt and 20 g olive oil. Pinch and fold until incorporated and the dough smooths slightly.

Bulk ferment with folds: Over 2–3 hours at warm room temp, perform 3–4 sets of stretch-and-folds every 30 minutes. Dough should become elastic and puffy by ~50–75% rise.

Cold ferment: Lightly oil the bowl, cover, and refrigerate 8–24 hours for flavor and easier handling.

Pan and proof: Generously oil a 9x13-inch pan (or a well-oiled sheet pan for thinner focaccia). Tip in the dough and gently stretch. If it resists, rest 10 minutes and stretch again. Cover and proof until noticeably puffy, 45–90 minutes depending on room temp.

Make the cinnamon topping: Mix 60 g melted butter, 100 g brown sugar, 2–3 tsp cinnamon, a pinch of salt, and 1 tsp vanilla. Stir to a thick, spoonable paste.

Dimple and swirl: Heat oven to 425°F (220°C). Oil your fingertips and deeply dimple the dough. Spoon the cinnamon mixture in thin ribbons across the surface, then use a butter knife to gently swirl. Avoid pooling too much in one spot.

Bake: Drizzle a little olive oil around the edges for crispness. Bake 18–24 minutes, until golden with caramelized spots and an internal temp around 200°F (93°C). If browning too quickly, tent with foil near the end.

Glaze: Whisk 100 g powdered sugar with 1–2 tbsp milk or cream and 1/2 tsp vanilla until smooth and pourable. Cool focaccia 10–15 minutes, then drizzle. Finish with a pinch of flaky salt if you like.

Slice and serve: Cut into squares or long sticks. Best slightly warm.