Feed your sourdough starter 4–8 hours before mixing by combining equal parts starter, flour, and water by weight. Leave it at room temperature until bubbly, risen to peak, and passing the float test (a small spoonful should float in water).
In a large mixing bowl, combine the bread flour and lukewarm water and mix until no dry flour remains. Cover and rest for 30–60 minutes to allow the flour to fully hydrate and begin developing gluten.
Add the active starter, raw honey, and whole milk powder (if using) to the dough and squeeze everything together with your fingers until fully incorporated with no streaks remaining. Cover and rest for 30 minutes.
Sprinkle the sea salt over the dough and work it in thoroughly, then add the softened butter a little at a time, folding and pressing until it is completely absorbed and the dough looks smooth and pulls away from the bowl. This takes about 5–8 minutes.
Over the next 2–3 hours, perform 4 sets of stretch and folds spaced 30 minutes apart: grab one side of the dough, stretch it up, fold it over, rotate the bowl 90°, and repeat four times per set. After the final set, cover and let the dough rest until it has grown 50–75%, looks puffy, and feels airy.
Turn the dough onto a lightly floured surface and fold the edges toward the center, rotating to build surface tension into a smooth round ball. Let it rest uncovered for 20–30 minutes.
Flip the dough smooth-side down, fold the sides to the center again, then roll it toward you to create a tight round or cylinder shape. Pinch the seam closed at the bottom.
Place the shaped dough seam-side up into a floured banneton, cover loosely with plastic wrap, and refrigerate for 8–16 hours (overnight is ideal). Bake directly from the refrigerator — do not let it come to room temperature first.
Place the Dutch oven with its lid inside the oven and preheat to 500°F (260°C) for at least 45–60 minutes. The Dutch oven must be fully preheated before the bread goes in.
Flip the cold dough onto parchment paper, score the top with a bread lame at a 30–45° angle, then carefully lower it into the screaming-hot Dutch oven. Bake covered at 500°F for 20 minutes, then remove the lid, reduce heat to 450°F (230°C), and bake for another 20–25 minutes until deep golden brown and the internal temperature reaches 205–210°F (96–99°C).
Transfer the baked loaf immediately to a wire cooling rack and wait at least 1 hour before slicing — the interior continues to set as it cools, and cutting too early results in a gummy crumb.