1. 1. Combine Wet Ingredients: In a large mixing bowl, whisk together the active sourdough starter, warm milk, granulated sugar, egg, and melted butter until well combined. The mixture should look slightly frothy from the starter.

2. 2. Add Dry Ingredients and Mix Dough: Gradually add the all-purpose flour and salt to the wet ingredients. Mix with a wooden spoon or your hands until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead for 5-7 minutes until it becomes smooth and elastic. It should spring back slightly when poked.

3. 3. First Rise (Bulk Fermentation): Place the kneaded dough into a lightly oiled bowl, turning it once to coat. Cover the bowl with plastic wrap or a damp kitchen towel and let it rise in a warm place for 4-8 hours, or until doubled in size. Alternatively, for a slower, deeper flavor development, you can place it in the refrigerator for 8-12 hours.

4. 4. Prepare the Filling and Shape the Dough: Once the dough has doubled, gently punch it down. On a lightly floured surface, roll the dough into a large rectangle, approximately 12×18 inches. In a small bowl, combine the brown sugar and cinnamon. Spread the softened butter evenly over the rolled-out dough, leaving a small border around the edges. Sprinkle the brown sugar and cinnamon mixture generously over the butter.

5. 5. Roll and Place in Loaf Pan: Starting from one of the long sides, tightly roll the dough into a log. Pinch the seam closed to seal. Carefully transfer the rolled dough into a lightly greased 9×5-inch loaf pan, seam-side down.

6. 6. Second Rise (Proofing): Cover the loaf pan with plastic wrap or a damp towel and let it rise in a warm place for 1-2 hours, or until the dough has nearly doubled in size and looks puffy. It should barely spring back when gently poked.

7. 7. Bake the Loaf: Preheat your oven to 375 F (190 C) during the last 15 minutes of proofing. Bake the bread for 30-35 minutes, or until the top is golden brown and a wooden skewer inserted into the center comes out clean. If the top browns too quickly, you can loosely tent it with aluminum foil.

8. 8. Cool Before Slicing: Once baked, remove the loaf from the oven and let it cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely. Slicing too early can result in a gummy texture.