Make Levain:Mix together 50g active sourdough starter, 50g water, and 50g bread flour to a jar. Cover and let ripen in a warm location for about 5 hours or until the levain is doubled, bubbly, and active.
Mix Ingredients:Once the levain is ready, add all of the sourdough sandwich bread dough ingredients to the bowl of a stand mixer including all of the levain.Attach the dough hook and mix together on medium speed until all of the ingredients come together into a shaggy dough.*Cover with a towel and let rest 10 minutes for the dough to fully hydrate.Continue mixing on medium speed for about 7-10 more minutes until the dough is tacky, not sticky and you can stretch a piece of the dough into a thin layer without it tearing (the windowpane test).
Bulk Fermentation (First Rise):Place the dough in a lightly oiled medium-sized bowl and cover.In a warm location, the dough should bulk ferment for about 4.5-5 hours. At the end of bulk fermentation, the dough should be doubled, domed, and appear full of air.
Overnight Proof:Place the bowl of dough into the refrigerator to proof overnight or up to 48 hours.
Shape the Dough:The next day, remove the dough from the refrigerator and butter a 8.5"x4.5" bread loaf pan.Turn out the dough onto a lightly floured surface and lightly press the dough into a square shape with your fingers.Using a rolling pin, evenly roll out the dough into a large oval (about 8.5" wide and 17" long).Starting at the short end closest to you, use your hands to roll up the dough into a log. Pick up the dough log and place it seam-side down into the bread loaf pan.
Final Proof:Cover the bread loaf pan and place in a warm location to proof for 4.5-5 hours.Final proof is complete when the dough looks swollen, has filled up the bread loaf pan, and rises about 1 inch above the bread loaf pan.If you press the dough with a lightly floured finger, it should leave a slight indentation.
Bake:Preheat the oven to 375ºF (190ºC).Brush the top of the bread dough with a tablespoon of melted butter.Bake the sourdough sandwich bread for 35-40 minutes. The sandwich bread is done when the top crust is evenly and lightly browned and the internal temperature reads between 190-200ºF (88-93ºC).Let cool on a wire rack for about 5 minutes. Turning the still hot bread out of the bread loaf pan to cool completely on a wire rack. Slice and enjoy!