Combine flour, butter, sugar, and salt in a stand mixer (paddle attachment). Mix until the butter looks crumbly.

Add starter and warm water. Mix until the flour is fully incorporated (finish with your hands if needed). Cover and let rest for 30 minutes.

Switch to the dough hook and knead on medium-low for 6-8 minutes until soft and elastic. If sticky, add a dusting of flour.

By Hand: Knead on a floured surface for 8-10 minutes until smooth and elastic.

Cover the bowl and let rise overnight at room temperature (68°F) until doubled in size, about 10-12 hours.

Grease an 8.5×4-inch loaf pan.

Flatten the dough to release air bubbles, then roll into a log. Rest for 5-10 minutes. Tighten the shape by cupping the dough and place it in the pan, seam side down.

Cover with lightly oiled plastic wrap and let rise until the dough is 1 inch above the rim of the pan, about 1 1/2-2 hours.

Preheat oven to 375°F (or 500°F for better oven spring, reducing to 375°F when the loaf goes in).

Bake on the center rack for 45-50 minutes, or until golden brown.

Cool in the pan for 10 minutes, then transfer to a rack to cool completely. Store in a plastic bag at room temperature for up to 3 days.