In a large bowl, whisk warm water with sourdough starter until fully dissolved. Add 25 g granulated sugar and whisk to dissolve. Incorporate flour using a wooden spoon or dough whisk until shaggy, then switch to hands for final mixing. Sprinkle kosher salt on the dough surface after 5 minutes.
Cover with plastic wrap or a shower cap and let rest 30 minutes. Use damp hands to pinch salt into the dough for 2 minutes. Shape into a round ball and repeat the rest for another 30 minutes.
Gently stretch and fold dough 4 times (every 30 minutes). After the final fold, let bulk ferment 3-4 hours until puffy and bubbly.
On a floured surface, stretch dough into a 10×15″ rectangle. Sprinkle with cinnamon and brown sugar (leaving ½” border). Fold top third over the center, then bottom third over the layers. Add final cinnamon-brown sugar layer and roll tightly into a log. Shape into a round boule.
Proof in a floured banneton (lined with tea towel) in the fridge for 2-3 hours. Preheat oven with Dutch oven at 450°F for 30 minutes. Bake covered for 25 minutes, then uncovered for 18-20 minutes until mahogany-brown.