In a large bowl, combine 295 grams room temperature water and 95 grams plain Greek yogurt. Whisk until mostly smooth.

Add 375 grams bread flour and 125 grams Kamut flour. Mix until no dry flour remains. The dough will be shaggy and that's ok. Cover and rest for 30 minutes.

If you want to add the optional protein powder, add it now with the flour.

Add 100 grams active sourdough starter and 13 grams salt. Mix until fully incorporated. To do this, dimple and pinch the dough with your fingers to help distribute the starter and salt evenly.

Once incorporated, mix the dough vigorously for 6–8 minutes until it becomes smoother and more cohesive. Cover and let the dough rest for 30 minutes.

Perform 4 rounds of stretch and folds, spaced 30 minutes apart. To do this: Wet your hands so the dough doesn’t stick. Lift one side of the dough, stretch it upward, and fold it over the center. Rotate the bowl and repeat until all four sides have been folded. After each round, cover the dough and let it rest 30 minutes.

After the fourth round, transfer the dough to a bulk fermentation container with tall straight sides. Cover the dough and let it continue bulk fermenting.

Let the dough rise at room temperature until it becomes light & airy, has a slightly domed top, has almost doubled in size and you can see lots of bubbles forming along the sides of the bowl. This can take 4–8 hours.

Turn the dough out onto your work surface and pre-shape into a round. Let the dough rest uncovered for 15 minutes.

After resting, flip the dough over so the smooth side is facing down. Shape the dough into a tight, smooth ball.

Place the dough into a floured banneton basket, with the seam facing up. Cover and refrigerate for 8–18 hours to cold ferment.

Preheat your oven to 450°F (232°C) with a Dutch oven inside for 30 to 45 minutes.

Turn your cold dough out onto parchment paper or a bread mat and score it however you like.

Carefully transfer the dough to the hot Dutch oven, cover, and bake for 20 minutes.

Uncover and bake for another 15 – 20 minutes, or until the crust is deep golden brown and the internal temperature reaches at least 205°F.

Let your loaf cool for one to two hours before slicing.