Mix Dough – In a stand mixer, combine flour, butter, sugar, and salt. Mix until butter resembles fine crumbs. Add sourdough starter and warm water. Mix until combined. Cover and let rest for 30 minutes.
Knead – Switch to a dough hook and knead for 6-8 minutes until the dough is soft, stretchy, and no longer sticky.
Bulk Rise – Cover the bowl and let the dough rise overnight (10-12 hours) until doubled in size.
Shape Dough – Lightly grease a loaf pan. Flatten the dough to release air bubbles, then roll it into a log and place in the pan.
Second Rise – Cover and let rise again until the dough reaches about 1 inch above the pan rim.
Bake – Preheat oven to 375°F. Bake for 45-50 minutes until golden brown. Let cool for at least 1 hour before slicing.