Mix the flour, starter, and water together until a rough, shaggy dough forms. Cover and let rest for 1 hour.
Add the salt using wet hands, working it into the dough until fully incorporated.
Perform stretch and folds every hour for 3 hours, then let the dough bulk ferment covered at room temperature until it has increased in size by 50 to 70 percent and jiggles when gently shaken. Anywhere from 6-12 hours depending on the temperature and strength of your dough and starter.
Lightly flour your work surface, turn the dough out, and pre shape it into a round. Let it rest seam side down for 20 to 30 minutes.
Shape the dough into a batard, place it seam side up in a banneton, and cover. Cold proof in the refrigerator for 8 to 12 hours.
When you are ready to bake, stir together the brown sugar, granulated sugar, cinnamon, and cornstarch in a small bowl to make the filling.
Turn the cold loaf out onto parchment, top side down.
Slice down through the loaf vertically.
Then go back through those slices with kitchen scissors to cut them into sections, opening up the interior.
Sprinkle the cinnamon sugar filling generously down into all of the cuts, distributing it evenly so no single spot gets a thick pile.
Starting at one end, gently roll the loaf up into a log, taking care not to deflate it. Tuck the bottom edge underneath to seal.
Set the rolled loaf on parchment and lower it into a parchment lined 10 by 5 inch loaf pan. Place a second loaf pan upside down on top to create a mini Dutch oven.
Bake at 450 degrees Fahrenheit for 9 minutes with the lid on.
Remove from the oven, take off the top pan, and score one line down the center of the loaf. Replace the lid and bake for 30 minutes.
Remove the lid and bake for a final 10 minutes, or until the internal temperature reaches 190 degrees Fahrenheit. Because the sugar makes this loaf brown early, use a thermometer rather than relying on color.
Let the loaf cool for 3 to 4 hours before slicing.