1. Prepare the Dough (Evening): In a large mixing bowl, combine the active sourdough starter and warm water. Stir until the starter is mostly dissolved. Add the bread flour, salt, and sugar. Mix with a wooden spoon or your hands until a shaggy dough forms and no dry flour remains. It will be quite sticky. Cover the bowl with a damp kitchen towel or plastic wrap and let it rest for 30 minutes at room temperature. This is called autolyse, and it helps the flour hydrate.

2. Incorporate the Butter and Knead Lightly: After the rest, add the softened unsalted butter to the dough. Using your hands, gently work the butter into the dough until it’s fully incorporated. The dough might feel a bit messy at first, but keep folding and squeezing until the butter is absorbed. Then, perform a few sets of “stretch and folds” in the bowl: grab a portion of the dough, stretch it up, and fold it over itself. Rotate the bowl and repeat 4-6 times. This builds gluten strength.

3. Overnight Bulk Fermentation: Lightly oil a clean large bowl with olive oil. Transfer the dough to the oiled bowl, turning it once to coat. Cover the bowl tightly with plastic wrap or a lid. Place the bowl in the refrigerator for 8-12 hours, or overnight. This slow, cold fermentation develops flavor and makes the dough easier to handle.

4. Shape the Loaf (Morning): The next morning, remove the dough from the refrigerator. It will have risen slightly and feel bubbly. Gently turn the dough out onto a lightly floured surface. Gently pat it into a rectangle. Fold the top third of the dough down to the center, then fold the bottom third up over that. Rotate the dough 90 degrees, then roll it up tightly from one end to the other, creating a log shape. Pinch the seam closed.

5. Final Proofing: Grease a standard 9×5-inch loaf pan. Carefully place the shaped dough seam-side down into the prepared loaf pan. Cover the loaf pan loosely with plastic wrap or a damp kitchen towel. Let it proof at room temperature for 1-2 hours, or until it has visibly increased in size and looks puffy. You can test if it’s ready by gently poking it with a floured finger; if the indentation slowly springs back, it’s ready to bake.

6. Bake the Bread: Preheat your oven to 375 F (190 C) with a rack in the middle. Once the oven is preheated and the dough is proofed, place the loaf pan in the oven. Bake for 35-40 minutes, or until the crust is golden brown and the internal temperature reaches 200-210 F (93-99 C) when measured with an instant-read thermometer. If the top is browning too quickly, you can loosely tent it with aluminum foil.

7. Cool Completely: Remove the loaf from the oven and immediately turn it out onto a wire cooling rack. This is crucial to prevent the bottom from becoming soggy. Let the bread cool completely for at least 1-2 hours before slicing. Slicing hot bread can lead to a gummy texture.