In the bowl of your stand mixer, combine the active sourdough starter and warm water. Whisk gently until the starter is mostly dispersed.
Add the bread flour and salt to the bowl. If using, add the softened butter. Mix on low speed with the dough hook attachment until a shaggy dough forms and no dry flour remains. This should take about 2-3 minutes.
Increase the speed to medium-low and knead for 8-10 minutes, or until the dough is smooth, elastic, and pulls away from the sides of the bowl. It should pass the windowpane test (a small piece of dough can be stretched thin enough to see light through without tearing).
Transfer the dough to a lightly oiled bowl or container. Cover and let it rest at room temperature for 4-6 hours, or until it has increased in volume by about 50-75% and shows signs of fermentation (bubbles, a slightly domed top). Perform a few sets of stretch and folds during the first 2 hours if desired.
Gently turn the dough out onto a lightly floured surface. Shape it into a loaf that will fit your loaf pan. Be careful not to deflate the dough too much. Tuck the seams underneath to create surface tension.
Place the shaped dough into a greased 9x5 inch loaf pan. Cover loosely with plastic wrap or a damp towel.
Allow the dough to proof at room temperature for another 2-4 hours, or until it has risen about 1 inch above the rim of the pan. Alternatively, you can cold proof it in the refrigerator overnight for a more developed flavor.
Preheat your oven to 375°F (190°C) about 30 minutes before you plan to bake. If you cold-proofed, let the dough sit at room temperature for about 30-60 minutes before baking.
Bake for 35-45 minutes, or until the crust is a deep golden brown and the internal temperature of the bread reaches 200-210°F (93-99°C) on an instant-read thermometer. If the crust is browning too quickly, you can tent it loosely with aluminum foil.
Once baked, immediately remove the bread from the loaf pan and place it on a wire rack to cool completely before slicing. This is crucial for the texture.
