In a large mixing bowl, combine the active sourdough starter, warm water, and honey; mix until the honey dissolves.

Add the flour and salt and mix until a shaggy dough forms with no dry flour remaining.

Cover the bowl and let the dough rest at room temperature for 30 minutes.

Perform a series of stretch-and-folds: wet your hands, grab an edge, stretch up, fold over the center, rotate 90 degrees and repeat a total of four times.

Cover and let rise 4–6 hours at room temperature until doubled and puffy with bubbles.

Place the dough in the refrigerator for 8–12 hours (or overnight) to develop flavor.

The next day, let the dough sit at room temperature for 1–2 hours.

Line a 9×13 inch baking sheet with parchment, then gently stretch the dough to fit, working from the center outward; if it keeps springing back, let it rest for 10 minutes.

Cover and let dough rise another 2–3 hours until puffy.

Preheat oven to 400°F (200°C).

Mix melted butter, brown sugar, and cinnamon into a thick paste; drizzle over the dough, then use your fingers to create deep dimples and swirls across the surface.

Bake for 20–25 minutes until golden and hollow-sounding when tapped.

Whisk powdered sugar, milk, and vanilla until smooth, let focaccia cool for 10 minutes, then drizzle glaze and serve warm.