Add the flour, softened butter, sugar, and salt to your mixer bowl fitted with the paddle attachment. Mix on low until the butter breaks down into small crumbs throughout the flour. This only takes a minute or two.
Pour in your active starter and warm water, then mix until everything comes together into a shaggy, slightly sticky dough. Cover the bowl and let it rest for 30 minutes before kneading.
Switch to the dough hook and knead on medium-low speed for 6 to 8 minutes, until the dough turns soft and smooth. It shouldn't stick to your hands anymore; if it does, add just a light dusting of flour.
No stand mixer? Knead by hand on a floured surface for about 8 to 10 minutes. Focus on how the dough feels rather than counting minutes exactly.
Cover the bowl tightly with plastic wrap and leave it at room temperature, ideally around 68°F, for 10 to 12 hours. The dough is ready once it looks puffy and has roughly doubled in size.
Warmer kitchens speed this up, while cooler ones slow it down, so watch the dough itself rather than the clock.
Turn the risen dough onto a lightly floured surface and gently press it flat to release the larger air bubbles. Roll it into a tight log, tucking the ends underneath as you go.
With floured hands, pull the dough toward you a few times to build surface tension, then place it seam-side down into a buttered loaf pan.
Cover the pan loosely with oiled plastic wrap and let the dough rise until the center domes about an inch above the rim, which usually takes 1.5 to 2 hours or longer in a cooler kitchen. You're not aiming for double in size here.
Preheat your oven to 375°F (or 500°F, then lower it once the bread goes in for extra oven spring). Bake for 45 to 50 minutes until deeply golden, then cool in the pan briefly before moving the loaf to a wire rack. Let it rest for about an hour before slicing.