1. Prepare the Dough: In a large mixing bowl, combine the active sourdough starter, warm milk, 1/4 cup granulated sugar, melted butter, and egg. Whisk until well combined. Gradually add the flour and salt, mixing 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’s smooth and elastic. It should spring back slowly when poked.

2. First Rise (Bulk Fermentation): Place the kneaded dough in a lightly oiled bowl, turning it once to coat. Cover the bowl tightly with plastic wrap or a damp kitchen towel. Let it rise at room temperature for 8-12 hours, or until it has doubled in size. This can be done overnight or during the day. The dough should look visibly puffy and have small bubbles on the surface.

3. Prepare the Filling: While the dough is on its second rise, or just before you’re ready to shape, prepare the filling. In a small bowl, cream together the softened butter, brown sugar, and ground cinnamon until well combined and spreadable.

4. Roll and Fill the Dough: Once the dough has completed its first rise, gently punch it down to release the air. On a lightly floured surface, roll the dough into a large rectangle, approximately 12×18 inches and about 1/4-inch thick. Spread the cinnamon sugar filling evenly over the entire surface of the dough, leaving a small border on one of the long edges.

5. Shape the Rolls: Starting from the long edge opposite the border, carefully roll the dough into a tight log. Pinch the seam closed to seal the roll. Using a sharp knife or unflavored dental floss, cut the log into 12 equal slices, about 1.5 inches thick.

6. Second Rise (Proofing): Arrange the cut rolls in a lightly greased 9×13 inch baking dish, leaving a little space between each roll. Cover the dish with plastic wrap or a clean kitchen towel. Let the rolls rise in a warm place for 1-2 hours, or until they are visibly puffy and have nearly doubled in size. They should look soft and pillowy.

7. Bake the Rolls: Preheat your oven to 375 F (190 C). Remove the cover from the rolls and bake for 25-30 minutes, or until they are golden brown on top and cooked through. The internal temperature should reach about 200 F (93 C).

8. Prepare the Cream Cheese Frosting and Frost: While the rolls are baking or cooling slightly, prepare the frosting. In a medium bowl, beat together the softened cream cheese and butter until smooth and creamy. Gradually add the powdered sugar, vanilla extract, and 1 tablespoon of milk or cream, beating until light and fluffy. Add more milk or cream, a teaspoon at a time, until the frosting reaches your desired consistency. Once the rolls are slightly cooled but still warm, spread the frosting generously over them.