1. Prepare the Dough Base: In a large mixing bowl, combine the active sourdough starter, warm milk, 1/4 cup granulated sugar, melted butter, and egg. Whisk these wet ingredients together until well combined. This initial mix helps to activate the starter and dissolve the sugar.

2. Mix in Dry Ingredients: Add the salt and 2 cups of the all-purpose flour to the wet mixture. Mix with a wooden spoon or a dough whisk until a shaggy dough forms. Gradually add the remaining 2 cups of flour, mixing until the dough comes together and pulls away from the sides of the bowl. It might still be a bit sticky.

3. Knead the Dough: Turn the dough out onto a lightly floured surface. Knead for 8-10 minutes until the dough is smooth, elastic, and no longer sticky. If it’s too sticky, add a tiny bit more flour, but be careful not to add too much. Form the dough into a ball.

4. First Rise (Bulk Fermentation): Lightly grease a clean bowl with butter or oil. Place the dough ball in the bowl, turning it once to coat. Cover the bowl with plastic wrap or a damp kitchen towel. Let it rise in a warm spot for 4-8 hours, or until doubled in size. Alternatively, you can place it in the refrigerator for 8-12 hours (or overnight) for a slower, more flavorful rise.

5. Shape the Rolls: Once the dough has doubled, gently punch it down and turn it out onto a lightly floured surface. Roll the dough into a large rectangle, approximately 12×18 inches. In a small bowl, mix together 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 brown sugar-cinnamon 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 it. Using a sharp knife or unflavored dental floss, cut the log into 12 equal slices, about 1.5 inches thick.

7. Second Rise (Proofing): Arrange the sliced rolls in a buttered 9×13-inch baking dish, leaving a little space between them. Cover the dish loosely with plastic wrap or a damp towel. Let them rise in a warm spot for 1-2 hours, or until visibly puffy and almost doubled in size. They should look soft and pillowy.

8. Bake the Cinnamon Rolls: Preheat your oven to 375 F (190 C). Once preheated, remove the cover from the rolls and bake for 25-30 minutes, or until golden brown on top and cooked through. The internal temperature should be around 200-210 F.

9. Prepare the Cream Cheese Frosting: While the rolls are baking or cooling slightly, prepare the frosting. In a medium bowl, beat together the softened cream cheese and softened butter until smooth and creamy. Gradually add the powdered sugar, mixing until well combined. Stir in the vanilla extract and 2-3 tablespoons of milk or cream, one tablespoon at a time, until the frosting reaches your desired consistency – smooth and spreadable.

10. Frost and Serve: Once the cinnamon rolls are out of the oven, let them cool in the pan for about 10-15 minutes. This allows them to set slightly. Then, generously spread the cream cheese frosting over the warm rolls. Serve immediately and enjoy!