In a large mixing bowl, combine the sourdough starter, warm water, melted butter, sugar, and salt. Stir until well mixed.
Gradually add the all-purpose flour, one cup at a time, mixing until the dough comes together and is no longer sticky.
Knead the dough on a lightly floured surface for about 5-7 minutes until smooth and elastic.
Place the kneaded dough back into the mixing bowl and cover it. Let it rise in a warm place for 1-2 hours, or until it has doubled in size.
Preheat your oven to 375°F (190°C).
After the dough has risen, punch it down gently and shape it into a loaf. Place the shaped dough into the greased loaf pan.
Cover the pan with a kitchen towel and let the bread rise again for about 30-45 minutes, or until it has risen above the rim of the pan.
Bake the bread in the preheated oven for 25-30 minutes, or until the top is golden brown and the bread sounds hollow when tapped on the bottom.
Remove from oven and allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.