Prepare the Dough: In a large mixing bowl, combine the active sourdough starter, lukewarm milk, and granulated sugar. Mix until the sugar dissolves. Add the melted butter and egg, stirring until well combined.

Mix in Dry Ingredients: Gradually add the all-purpose flour and salt to the wet mixture. Stir until a soft dough forms.

Knead the Dough: Transfer the dough onto a floured surface and knead for about 5-7 minutes until it becomes smooth and elastic.

First Rise: Place the dough in a greased bowl, cover with a damp cloth, and let it rise in a warm place for approximately 8-12 hours, or until doubled in size.

Prepare the Filling: In a small bowl, mix the softened butter, brown sugar, and ground cinnamon until well combined.

Roll Out the Dough: Once the dough has risen, punch it down and roll it out on a floured surface into a rectangle approximately 12×18 inches in size.

Add the Filling: Spread the cinnamon-sugar mixture evenly over the rolled-out dough, leaving a small border around the edges.

Form the Rolls: Starting from the long edge, tightly roll the dough into a log. Cut the log into 12 equal pieces.

Second Rise: Place the rolls into a greased 9×13-inch baking dish, cover, and let them rise for about 1-2 hours, or until puffy.

Bake: Preheat the oven to 375°F (190°C). Bake the rolls for 25-30 minutes, or until golden brown.

Prepare the Frosting: While the rolls are baking, beat together the cream cheese and butter until smooth. Add the powdered sugar, vanilla extract, and milk, and continue to beat until the frosting is creamy.

Frost the Rolls: Once the rolls are out of the oven, let them cool slightly before spreading the cream cheese frosting over the top.