1. 1. Activate the Starter and Combine Wet Ingredients: In a large mixing bowl, combine your active sourdough starter, plain whole milk yogurt, warm water, and granulated sugar. Stir everything together until well combined. The mixture should look milky and slightly frothy from the starter.

2. 2. Add Dry Ingredients and Form the Dough: Add the salt and 5 cups of all-purpose flour to the wet mixture. Use a sturdy spoon or your hands to mix until a shaggy dough forms and no dry streaks of flour remain. If the dough is very sticky, add the remaining 1/2 to 1 cup of flour gradually, just until it comes together.

3. 3. Initial Kneading and Bulk Fermentation: Turn the dough out onto a lightly floured surface. Knead for about 5-7 minutes until the dough is somewhat smooth and elastic. It doesn’t need to be perfectly smooth yet. Lightly grease your mixing bowl with a little oil or butter, place the dough back in, cover it with plastic wrap or a damp kitchen towel, and let it rise at room temperature (around 70-75F) for 8-12 hours, or until it has doubled in size. This is your bulk fermentation, and it can be done overnight.

4. 4. Shape the Loaf: Once the dough has doubled, gently punch it down to release the air. Turn it out onto a lightly floured surface. Gently stretch the dough into a rectangle, then fold the top third down to the center and the bottom third up over the top, like a letter. Rotate the dough 90 degrees, then roll it up tightly from one end to the other to form a log. Pinch the seam closed.

5. 5. Second Proofing in the Loaf Pan: Grease a 9×5-inch loaf pan with butter. Place the shaped dough seam-side down into the prepared pan. Cover the pan loosely with plastic wrap or a damp towel and let it rise again in a warm spot for 2-4 hours, or until the dough has risen about 1 inch above the rim of the pan. It should look puffy and soft.

6. 6. Preheat Oven and Bake: While the dough is proofing for the second time, preheat your oven to 375 F (190 C). Once the loaf has proofed, brush the top with the melted butter. Bake for 35-45 minutes, or until the crust is golden brown and the internal temperature reaches 200-210 F (93-99 C) with an instant-read thermometer. If the crust is browning too quickly, you can loosely tent it with aluminum foil.

7. 7. Cool Completely Before Slicing: Once baked, remove the loaf from the oven and immediately turn it out onto a wire rack. Let it cool completely for at least 1-2 hours before slicing. Slicing too early can result in a gummy texture.