1. Activate Your Sourdough Starter and Combine Wet Ingredients: In a large mixing bowl, combine your active sourdough starter, warm milk, granulated sugar, egg, melted and cooled butter, and salt. Whisk these ingredients together until they are well combined and the sugar has dissolved. The mixture should look slightly milky and uniform.

2. Gradually Add Flour to Form the Dough: Start by adding 3 cups of the all-purpose flour to the wet ingredients. Mix with a wooden spoon or a dough hook on a stand mixer until a shaggy dough forms. Gradually add the remaining 1/2 to 1 cup of flour, a little at a time, until the dough comes together and is no longer overly sticky. You want a dough that is soft and pliable, but still a bit tacky to the touch. Avoid adding too much flour, as this can make your bread dry.

3. Knead the Dough Until Smooth and Elastic: Turn the dough out onto a lightly floured surface. Knead for 5-7 minutes by hand, or 3-5 minutes with a stand mixer on medium speed, until the dough is smooth, elastic, and passes the windowpane test (you can stretch a small piece thin enough to see light through it without tearing). This develops the gluten, which is essential for a good bread texture.

4. First Rise (Bulk Fermentation): Lightly grease a clean bowl with a little oil. Place the kneaded dough in the bowl, turning it once to coat all sides. Cover the bowl with plastic wrap or a clean kitchen towel and let it rise in a warm spot for 2-3 hours, or until it has visibly increased in size (it may not quite double, but should look puffy and expanded). The exact time will depend on your starter’s activity and room temperature.

5. Prepare the Cinnamon Swirl Filling and Shape the Loaf: While the dough is rising, prepare the filling. In a small bowl, combine the brown sugar and ground cinnamon. Once the dough has completed its first rise, gently punch it down to release the air. Turn it out onto a lightly floured surface and roll it into a large rectangle, approximately 12×18 inches. Spread the 2 tablespoons of softened butter evenly over the entire surface of the dough, leaving a small border along one of the long edges. Sprinkle the brown sugar and cinnamon mixture evenly over the buttered dough. Tightly roll the dough up from one of the long edges, creating a log. Pinch the seam closed to seal.

6. Second Rise (Proofing) and Preheat Oven: Carefully transfer the rolled dough log to a lightly greased 9×5-inch loaf pan. Cover the loaf pan loosely with plastic wrap or a clean kitchen towel. Let the dough rise in a warm spot for another 30-60 minutes, or until it looks puffy and has risen slightly above the rim of the pan. About 15 minutes before the end of the second rise, preheat your oven to 375 F (190 C).

7. Bake the Cinnamon Swirl Bread: Place the loaf pan in the preheated oven. Bake for 30-35 minutes, or until the top is golden brown and the internal temperature reaches 200-210 F (93-99 C) when measured with an instant-read thermometer. If the top starts to brown too quickly, you can loosely tent it with aluminum foil.

8. Cool Before Slicing: Once baked, remove the loaf pan from the oven and let the bread cool in the pan for 10-15 minutes. Then, carefully remove the loaf from the pan and transfer it to a wire rack to cool completely before slicing. This allows the internal structure to set and prevents a gummy texture.