Ensure your sourdough starter is active and bubbly, fed 4-12 hours prior and at its peak (doubled in size). In a large mixing bowl, combine the warm water and active sourdough starter. Whisk until the starter is mostly dissolved.

Add the bread flour and whole wheat flour to the bowl. Mix with your hands or a dough scraper until no dry spots remain and a shaggy dough forms. Cover the bowl with plastic wrap or a shower cap and let it rest for 30 minutes (autolyse). This allows the flour to fully hydrate.

After the autolyse, sprinkle the salt and sugar over the dough. Add the softened butter cubes. Using wet hands, knead the dough in the bowl for 5-7 minutes until the salt, sugar, and butter are fully incorporated and the dough starts to feel smoother and more elastic.

Cover the bowl again and let the dough rest for 30 minutes. Over the next 2-3 hours, perform 4 sets of stretch and folds, spaced 30 minutes apart. For each set, lightly wet your hands, grab a portion of the dough from one side, stretch it up, and fold it over the center. Rotate the bowl and repeat 3-4 times until all sides have been stretched and folded. Recover the dough after each set.

After the final set of stretch and folds, allow the dough to continue bulk fermenting at room temperature (72-76°F / 22-24°C) until it has increased in volume by about 50-75% and feels light and airy. This can take anywhere from 3 to 7 hours depending on your starter's strength and ambient temperature.

Lightly flour your work surface. Gently turn the dough out onto the surface. Preshape the dough into a loose rectangle or round. Let it rest, uncovered, for 15-20 minutes to relax the gluten.

Lightly grease a 9x5 inch loaf pan. To shape the loaf, gently flatten the dough into a rectangle. Fold the top third of the dough down to the center, pressing firmly. Fold the bottom third up and over the previous fold, creating a tighter rectangle. Then, starting from one short end, roll the dough tightly into a log. Pinch the seam closed. Place the shaped dough seam-side down into the prepared loaf pan.

Cover the loaf pan with plastic wrap or a clean kitchen towel. Proof at room temperature for 2-4 hours, or until the dough has nearly doubled in size and feels very puffy. Alternatively, for a more developed flavor and easier handling, cold proof in the refrigerator for 8-16 hours.

Preheat your oven to 400°F (200°C) with a rack in the center. If cold proofing, remove the dough from the fridge while the oven preheats.

Bake for 40-50 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210°F (93-99°C). If the crust is browning too quickly, you can loosely tent it with aluminum foil for the last 15-20 minutes.

Once baked, immediately remove the loaf from the pan and transfer it to a wire rack to cool completely before slicing (at least 2-3 hours). Slicing hot bread can result in a gummy texture.