In a large mixing bowl, combine the bread flour and water, mixing until no dry flour remains, then let it sit for 30 minutes.

Add the sourdough starter, salt, sugar, and softened butter to the flour mixture, and mix until a rough dough forms.

Knead the dough on a lightly floured surface for about 10 minutes until smooth and elastic.

Place the dough in a clean bowl, cover with plastic wrap or a damp cloth, and let it rest in a warm place for 3 hours, folding every 30 minutes.

After resting, shape the dough into a loaf and place it into a greased 9x5 inch loaf pan.

Cover the loaf pan and let the dough rise until it has doubled in size, about 1 hour.

Preheat the oven to 375°F (190°C).

Bake the bread for 35 minutes, or until the top is golden brown and the loaf sounds hollow when tapped.

Allow the bread to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing.