Mix the dough: In a large mixing bowl, combine the sourdough discard, water, flour, salt, sugar, and yeast. Mix until a shaggy dough forms. Add the melted butter and continue mixing until the dough starts coming together.
Knead the dough: Transfer the dough to a lightly floured surface and knead for about 8–10 minutes until it becomes smooth and elastic. If using a stand mixer, knead with the dough hook on medium speed for 5–7 minutes.
First rise: Place the dough in a greased bowl, cover with a damp towel, and let it rise in a warm place for about 1–1.5 hours, or until doubled in size.
Shape the dough: Gently deflate the dough and shape it into a log. Place it into a greased 9×5-inch loaf pan, seam side down.
Second rise: Cover the pan and let the dough rise for another 45–60 minutes, or until it puffs up just above the rim of the pan.
Bake: Preheat the oven to 375°F (190°C). Bake for 30–35 minutes, or until the top is golden brown and the loaf sounds hollow when tapped.
Cool and enjoy: Let the bread cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing. Enjoy!