Prepare the Levain: In a medium bowl, add flour, sourdough starter, and water. Using a wooden spoon, mix to combine and then transfer into a clean jar. Place a rubber band at the starting point to track its growth. Place the lid on top without sealing. Let rest for 3 to 6 hours until it raises to 2 ½ times the original size.

Once the levain is ready, in a small bowl, add 1 ½ tablespoons (21g) of the water and salt. Stir until the salt dissolves and set aside. In a large bowl, add the levain and the remaining water and whisk until everything is incorporated. Add flour and sugar. Shaping your hand like a claw, bring the ingredients together, mixing and periodically squeezing the dough together. Continue this process for 3 to 5 minutes until everything is fully incorporated. The dough will still be sticky but it will come together after the folding process. Cover the bowl with a kitchen towel or plastic wrap and let it rest for 1 hour at room temperature. At the end of this time, pour the dissolved salt over the dough. Knead by gently stretching and folding the dough until all of the liquid is incorporated and the texture is smooth. Cover the bowl once again and let rest for 40 minutes.

At the end of this time, lift the dough up from the middle, stretching and folding it over itself. Turn the bowl and repeat this process so each edge is incorporated back into the middle of the dough. Cover once again and let rest for 40 minutes. Repeat this resting and folding process three more times, but on each of the last two folds, add half of the cold grated butter into the dough as in the IG reel and let rest in the fridge for the 40 minute resting periods.

After chilling for the last time, lightly flour a work surface and transfer the dough onto the middle. Stretch the dough into a rectangular shape. Place the chocolate batons evenly across the dough. Begin shaping the dough by folding the long edges into the middle over each other. Then, roll the dough toward yourself, slightly stretching it and pressing the inside of the dough lightly to form a cylinder shape with the open edge underneath the dough. If you are using a circular proofing basket, make the loaf round by placing your hands on either side of the dough with the edges of your palms against the surface. Holding your left hand as a guide, use your right hand to push the outer edges of the dough lightly underneath while turning slightly. Continue this motion one or two times until lightly rounded. Carefully lift the ball onto your left hand, maintaining the dough shape, and place in the proofing basket with the open edge up. Lightly pull the seam edges together and pinch to seal. Let sit at room temperature for 10 to 15 minutes. Cover the dough completely with plastic wrap or a plastic grocery bag and keep it in the refrigerator overnight, 12 to 14 hours.

The next day, place a Dutch oven in the oven and preheat to 450F for at least 15 to 20 minutes. Remove from the fridge. Line a work surface with parchment paper. Flip the basket onto the parchment paper so the seam edge is down. Holding a sharp razor at a 45 degree angle, score the surface of the dough ½ inch deep. Add additional small cuts in a pattern to your preference. Immediately remove the Dutch oven from the oven and transfer the bread to the preheated pan with the parchment paper. Cover with a lid and bake for 30-35 minutes. Then remove the lid and bake another 15 to 20 minutes until deeply golden brown. If you have a kitchen thermometer, the bread should reach an internal temperature of around 200F. Remove from the Dutch oven and cool on a rack for 30 to 40 minutes before slicing.