In bowl of a stand mixer that is fitted with the paddle attachment, combine the flour, butter, honey or sugar, and salt. Mix on low speed until the butter resembles coarse crumbs.
Add the sourdough starter and warm water. Mix until no dry flour remains, you may need to finish mixing by hand. The dough should feel sticky. Cover the bowl with plastic wrap or a damp towel and let the dough rest for 30 minutes.
Switch to the dough hook attachment and mix on low (speed 2 on a KitchenAid) for 6–8 minutes. This is a 60% hydration dough, so the dough should feel sticky (SEE TIPS). Place the dough in a greased bowl. Use wet hands, a rubber spatula, or a bowl scraper to do so.
Cover the bowl with plastic wrap or a damp tea towel, and let the dough rise at room temperature (around 68°F) for 10–12 hours, or until doubled in size.
In the morning, grease an 8.5×4" loaf pan with butter. Turn the dough onto a floured surface and gently flatten it into a rectangular shape.
Roll into a log, tucking the ends underneath, using a bench scraper if you need to. Let the dough rest for about 5–10 minutes.
Then, using floured hands, gently tighten the shape by cupping and pulling it toward you. Place the dough seam side down in the prepared bread pan.
Cover loosely with oiled or buttered plastic wrap. Let the bread rise at room temperature until the dough has puffed about 1 inch above the pan’s rim (this usually takes 1½–2 ½ hours, but can vary with room temp and starter strength).
Preheat your oven to 375°F. Bake for 40–50 minutes, or until golden brown.
Let it cool in the pan for 10 minutes, then transfer to a wire rack to cool for at least 1 hour before slicing.