Cover the dough and leave it at room temperature. During the first two hours, perform three stretch-and-fold sessions at 30-minute intervals to build structure. The dough should rise by about half, feel lighter, and hold shape without becoming overly puffy.

Stir brown sugar and ground cinnamon together until evenly combined. Melt the butter separately and keep it warm. Preparing the filling ahead ensures quick shaping without deflating the dough later.

Turn the fermented dough onto a lightly floured surface and stretch it into a rectangle. Brush melted butter evenly across the surface, then scatter the cinnamon sugar mixture, leaving space near the edges. Roll the dough tightly into a log, sealing the seam underneath to hold the swirl during baking.

Place the shaped loaf into a floured banneton or loaf tin. Cover and refrigerate overnight. This slow proof strengthens flavour, firms the dough for baking, and keeps the cinnamon layers distinct rather than absorbed into the crumb.

Preheat the oven to 230°C. Bake the loaf with steam for the first 20 minutes to allow proper oven spring. Reduce the heat to 200°C and continue baking until the crust turns deep golden and the loaf sounds hollow when tapped underneath.

Transfer the baked loaf to a wire rack and allow it to cool completely. Cutting too early can cause the cinnamon sugar to smear and the crumb to compress, so patience here preserves both texture and appearance.