1. 1. Combine Starter and Water: In a large mixing bowl, gently whisk together the active sourdough starter and the warm water until the starter is mostly dissolved. The mixture should look milky and slightly frothy.

2. 2. Mix Dry Ingredients: Add the all-purpose flour, salt, and granulated sugar to the bowl. Use a Danish dough whisk or a sturdy spoon to mix until a shaggy dough forms and no dry flour remains. It will be sticky. Cover the bowl with a clean kitchen towel or plastic wrap and let it rest for 30 minutes (this is called autolyse, and it helps the flour hydrate).

3. 3. Incorporate Butter and Knead: After the rest, add the softened unsalted butter to the dough. Using your hands, knead the butter into the dough until fully incorporated. This can be a bit messy at first, but keep working it. Once the butter is in, continue to knead the dough for 5-7 minutes on a lightly floured surface or in a stand mixer with a dough hook until it becomes smooth, elastic, and passes the windowpane test (you can stretch a small piece thin enough to see light through it without tearing).

4. 4. First Fermentation (Bulk Rise): Lightly grease a clean bowl with olive oil. Place the kneaded dough into the oiled bowl, turning it once to coat. Cover the bowl and let it rise in a warm spot (70-75 F / 21-24 C is ideal) for 4-6 hours, or until it has nearly doubled in size. During this time, perform 2-3 sets of “stretch and folds” every 45-60 minutes during the first 2-3 hours of the bulk rise. To do this, gently pull a portion of the dough up and fold it over itself, rotating the bowl and repeating until all sides have been folded.

5. 5. Shape the Loaf: Gently turn the dough out onto a lightly floured surface. Gently pat it into a rectangle. Fold the top third down to the center, then fold the bottom third up over the top. Turn the dough over and gently roll it into a tight log, sealing the seam at the bottom. The goal is to create tension on the surface. Place the shaped dough seam-side down into a lightly greased 9×5-inch loaf pan.

6. 6. Second Fermentation (Proofing): Cover the loaf pan loosely with plastic wrap or a clean kitchen towel. Let it proof in a warm spot for 1.5-2.5 hours, or until the dough has risen to about 1 inch above the rim of the pan. It should feel light and airy.

7. 7. Bake the Bread: Preheat your oven to 375 F (190 C) about 20 minutes before the end of the proofing time. Once preheated, place the loaf pan in the center of the oven. Bake for 35-40 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210 F (93-99 C) when measured with an instant-read thermometer.

8. 8. Cool Completely: Remove the bread from the oven and immediately turn it out onto a wire rack. Allow it to cool completely for at least 1-2 hours before slicing. Slicing warm sourdough can result in a gummy texture.