1. Prepare the Dough: In a large mixing bowl, combine the active sourdough starter, warm milk, granulated sugar, salt, egg, and melted butter. Stir well until everything is thoroughly mixed. Gradually add 3 cups of the all-purpose flour, mixing until a shaggy dough forms. If the dough is too sticky, add the remaining 1/2 cup of flour, a little at a time, until it comes together. It should be soft but not overly sticky.

2. Knead and First Rise: Turn the dough out onto a lightly floured surface and knead for 5-7 minutes until it’s smooth and elastic. Place the dough in a lightly oiled bowl, turning it once to coat. Cover the bowl with plastic wrap or a damp towel and let it rise in a warm place for 8-12 hours, or until doubled in size. An overnight rise in the refrigerator can also work, allowing for a slower fermentation and deeper flavor.

3. Make the Filling: While the dough is rising (or after it’s risen), prepare the filling. In a small bowl, combine the softened 1/2 cup unsalted butter, 1/2 cup packed light brown sugar, and ground cinnamon. Mix until it forms a smooth paste.

4. Shape the Rolls: Once the dough has doubled, 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 cinnamon-sugar filling evenly over the entire surface of the dough, leaving a small border along one long edge.

5. Roll and Slice: Starting from the long edge opposite the border, tightly roll the dough into a log. Use a sharp knife or unflavored dental floss to slice the log into 12-15 equal-sized rolls, each about 1 to 1 1/2 inches thick.

6. Prepare the Caramel Topping and Second Rise: In a 9×13 inch baking dish, melt the 1/2 cup unsalted butter. Sprinkle the 1 cup packed light brown sugar evenly over the melted butter. Pour the heavy cream over the brown sugar and butter mixture. Arrange the sliced rolls cut-side down on top of the caramel mixture in the baking dish. Cover the dish loosely with plastic wrap or a clean towel and let the rolls rise again in a warm place for 1 hour, or until they look puffy and have nearly doubled in size.

7. Bake the Rolls: Preheat your oven to 375 F (190 C). Once the oven is preheated and the rolls have completed their second rise, remove the cover and bake for 25-30 minutes, or until the rolls are golden brown on top and the caramel is bubbly.

8. Finish and Serve: Carefully remove the baking dish from the oven. Let the rolls cool in the dish for 5-10 minutes. Then, place a large serving platter or baking sheet upside down over the baking dish and carefully invert the rolls onto the platter. Scrape any remaining caramel from the dish over the rolls. Drizzle with vanilla extract and a pinch of salt if desired. Serve warm and enjoy!