Combine 100 g sourdough starter, 100 g flour for stiff starter, 10 g sugar for stiff starter, and 40 ml water for stiff starter in a large mixing bowl until a thick, shaggy dough forms.

Cover the stiff starter and let it ferment until doubled, very bubbly, and able to float in water, about 4 to 6 hours.

Add 227 g active sourdough starter, 590 ml water, 1120 g all-purpose flour, melted butter, 2 tbsp sugar, and 1 tbsp salt to a large mixing bowl.

Mix the dough until all ingredients are fully combined and no dry flour remains.

Knead the dough with the stand mixer dough hook on medium speed for 10 minutes.

Knead the dough by hand for 2 additional minutes, until smooth, stretchy, and able to pass the windowpane test.

Coat a clean large mixing bowl with 1 tbsp neutral oil and place the dough inside.

Turn the dough over to coat it in oil, then cover the bowl.

Let the covered dough rise until doubled in size, about 480 to 720 minutes.

Gently deflate the risen dough and divide it into 2 equal portions.

Shape each dough portion into a tight loaf and grease 2 loaf pans.

Place one shaped loaf into each greased loaf pan and cover loosely with a kitchen towel.

Let the loaves rise until doubled and approximately 1 inch above the pan rims, about 120 min.

Preheat the oven to 425°F (220°C) while the loaves finish their final rise.

Bake the loaves at 425°F (220°C) for 10 minutes.

Reduce the oven temperature to 350°F (175°C) and bake for another 20 minutes until the crust is golden brown.

Check that the center of each loaf reaches 180 to 190°F (82 to 88°C), then remove the pans from the oven.

Remove the loaves from their pans and cool completely on a cooling rack before slicing.