1. Prepare the Dough Base: In a large mixing bowl, combine the warm milk, active sourdough starter, granulated sugar, melted butter, salt, and egg. Whisk these wet ingredients together until well combined. Gradually add the all-purpose flour, one cup at a time, mixing with a wooden spoon or your hands until a shaggy dough forms.

2. Knead and First Rise: Turn the dough out onto a lightly floured surface and knead for 5-7 minutes until it becomes smooth and elastic. It should spring back slightly when poked. Form the dough into a ball, place it in a lightly oiled bowl, turning once to coat, and cover with plastic wrap or a clean kitchen towel. Let it rise in a warm place for 2-3 hours, or until doubled in size. The dough should look puffy and feel airy.

3. Roll Out the Dough: Once risen, gently punch down the dough to release the air. Transfer it to a lightly floured surface and roll it into a large rectangle, approximately 12×18 inches, aiming for about 1/4 inch thickness. The edges should be relatively straight.

4. Add the Flavorful Filling: In a small bowl, mix together the ground cinnamon and packed light brown sugar. Spread the 2 tablespoons of softened butter evenly over the rolled-out dough, leaving a small border along one of the long edges. Sprinkle the cinnamon-sugar mixture over the butter, then evenly distribute the diced peaches over the cinnamon-sugar.

5. Roll and Slice the Rolls: Starting from one of the long edges, carefully roll the dough tightly into a log. Once rolled, pinch the seam closed to seal. Using a sharp knife or unflavored dental floss, cut the log into 12 even slices, about 1 1/2 inches thick.

6. Second Rise and Baking: Arrange the sliced 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 towel and let them rise in a warm spot for another 30-45 minutes, or until visibly puffy and almost touching. Preheat your oven to 375 F (190 C) during the last 15 minutes of the second rise.

7. Bake to Golden Perfection: Bake the cinnamon rolls for 25-30 minutes, or until they are golden brown on top and cooked through. The rolls should feel firm to the touch in the center.

8. Prepare and Frost: While the rolls are baking, prepare the cream cheese frosting. In a medium bowl, beat the softened cream cheese and 1/4 cup softened butter together until smooth and creamy. Gradually add the powdered sugar, mixing until combined. Stir in the vanilla extract and then add milk or cream, one tablespoon at a time, until the frosting reaches your desired consistency. Spread the frosting generously over the warm cinnamon rolls as soon as they come out of the oven.