1. In a large mixing bowl, gently mix your active sourdough starter with the warm water until the starter is mostly dissolved. The mixture will look milky and slightly frothy.

2. To the wet mixture, add the all-purpose flour, fine sea salt, and granulated sugar. Mix with a wooden spoon or your hands until a shaggy dough forms and all the flour is hydrated. It doesn’t need to be perfectly smooth at this stage.

3. Turn the shaggy dough out onto a lightly floured surface. Add the softened unsalted butter to the dough. Knead the dough for about 8-10 minutes 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. Place the kneaded dough into a lightly oiled bowl, turning it once to coat. Cover the bowl with plastic wrap or a damp kitchen towel and let it rise in a warm spot for 3-4 hours, or until it has doubled in size. The dough should look puffy and have some visible bubbles.

5. Gently turn the risen dough out onto a lightly floured surface. Gently deflate it and shape it into a tight log or oblong shape that will fit comfortably into your loaf pan. The goal is to create surface tension on the dough for a good rise.

6. Place the shaped dough into a greased 9×5-inch loaf pan. Cover the pan loosely with plastic wrap or a damp towel and let it rise again for 1-2 hours, or until it has nearly doubled in size and looks puffy. It should pass the “poke test” – when gently poked with a floured finger, the indentation should spring back slowly.

7. While the dough is on its second rise, preheat your oven to 375 F (190 C). Once the dough is ready, place the loaf pan in the preheated oven and bake for 35-40 minutes, or until the crust is golden brown and the internal temperature reaches 200-210 F (93-99 C) with an instant-read thermometer.

8. Remove the bread from the oven and immediately transfer it from the loaf pan to a wire rack to cool completely before slicing. This is crucial for the texture; slicing hot bread can make it gummy.