In a large mixing bowl, mix the active sourdough starter with water and salt until combined.

Add the flour and mix until a shaggy dough forms.

Let the dough rest for 1 hour covered with a damp tea towel (autolyse).

Perform 3 sets of 4 stretch and folds, spaced 30 minutes apart. (stretch and fold is just what it sounds like: Stretch the dough upward, fold in half, then give a quarter turn and repeat).

Cover the bowl with plastic wrap and let the dough rise for 6 hours to overnight for the first rise, depending on room temperature, until doubled in size.

In a small bowl, mix the butter, brown sugar, and cinnamon until smooth.

On a floured work surface, shape the dough into an approximately 8 × 12-inch rectangle, with the longer side closest to you.

Spread ⅔ of the cinnamon sugar mixture evenly across the surface.

Fold the bottom third of the rectangle up toward the center, then fold the top third down over it (like a letter).

Spread the remaining cinnamon-butter mixture over the exposed long rectangle.

Roll the dough up tightly, as if making a giant cinnamon roll, to form a large ball of dough.

Use a gentle push-pull motion to create surface tension and round the ball without tearing the dough (to keep the cinnamon swirl from oozing out the sides of the dough).

Place seam-side up into a flour-lined proofing basket.

Cover with plastic wrap or a damp towel and refrigerate for the second rise. This should be at least 2 hours or overnight for a cold ferment.

1 hour before baking, place a Dutch oven (with lid) into the oven and preheat to 475°F (245°C).

Flip the dough onto a piece of parchment paper and rub any excess flour into the top of the dough so it's in a smooth ball.

Score deeply with a dough lame (a baking razor blade, but a sharp knife works, too) at an angle to help create an ear.

Carefully remove the hot Dutch oven from the oven. Transfer the dough (using the parchment) into the pot.

Place 4–6 ice cubes under the parchment paper and replace the lid.

Let the bread cool for at least 1 hour before slicing.