1. Combine the wet ingredients: In a large mixing bowl, whisk together the active sourdough starter, warm milk, granulated sugar, room temperature egg, and melted and cooled butter until well combined. The mixture should be milky and slightly frothy from the starter.

2. Add the dry ingredients: 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. It will look a bit rough and sticky at this stage.

3. Knead the dough: Turn the dough out onto a lightly floured surface and knead for 8-10 minutes. The dough should become smooth, elastic, and less sticky. You’ll know it’s ready when it passes the windowpane test (you can stretch a small piece thin enough to see light through it without tearing).

4. First rise: Place the kneaded dough in a lightly oiled bowl, turning it once to coat. Cover the bowl with plastic wrap or a clean kitchen towel and let it rise in a warm place for 4-6 hours, or until doubled in size. This slow rise is crucial for flavor development.

5. Prepare the filling and shape the bread: 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 for the filling. Spread the softened butter evenly over the rolled-out dough, leaving a small border on one long edge. Sprinkle the cinnamon-brown sugar mixture evenly over the butter.

6. Roll and slice: Starting from one of the long edges, tightly roll the dough into a log. Pinch the seam closed to seal. Using a sharp knife or unflavored dental floss, cut the log into 12-15 equal slices, about 1 to 1.5 inches thick.

7. Second rise and bake: Arrange the slices snugly in a greased 9×13 inch baking dish. Cover loosely with plastic wrap or a kitchen towel and let rise in a warm place for another 1-2 hours, or until visibly puffy and almost doubled. Preheat your oven to 375 F (190 C). Bake for 30-35 minutes, or until golden brown on top and cooked through. The internal temperature should be around 200-210 F (93-99 C).

8. Glaze and serve: While the bread cools slightly, prepare the optional glaze by whisking together the powdered sugar, milk or water, and vanilla extract until smooth. Drizzle the glaze over the warm bread. Serve warm and enjoy!