Mix the dough: In a large bowl, whisk warm water and starter. Add flours and salt. Stir until no dry bits remain. Rest 20 minutes, then drizzle in olive oil and fold until incorporated. The dough will be sticky—this is good.

Strengthen the dough: Over 1 hour, perform 3 sets of stretch-and-folds every 20 minutes. Keep hands slightly wet to prevent sticking. The dough should grow smoother and bouncier.

Bulk ferment: Cover and let rise at room temperature until puffy and roughly doubled, 3–5 hours depending on warmth and starter strength. You should see bubbles along the sides.

Pan and cold proof: Generously oil a 9x13-inch pan. Gently transfer dough into the pan. Dimple lightly to spread, then rest 20 minutes. Repeat dimpling to fill the pan. Cover and refrigerate 8–12 hours or overnight for flavor and easier handling.

Make the cinnamon mix: Stir butter, brown sugar, cinnamon, salt, and vanilla into a smooth paste. If too firm, warm briefly until spreadable.

Final proof: Remove dough from the fridge and let sit 45–60 minutes until slightly puffy. Preheat oven to 425°F (220°C) with a rack in the center.

Add the swirl: Use oiled fingers to create shallow channels and dimples. Dollop the cinnamon paste across the surface. Gently spread and swirl with the back of a spoon, leaving a 1/2-inch border. Drizzle a little olive oil around the edges for crispness.

Bake: Bake 20–25 minutes until the top is deep golden with caramelized pockets. If browning too fast, tent loosely with foil for the last 5 minutes.

Glaze and cool: Whisk the glaze to a thick, pourable consistency. Cool focaccia 10–15 minutes, then drizzle over the warm surface. Slice or pull apart and serve.