In a large bowl, combine the sourdough starter, milk, melted butter, sugar, and egg. Mix well until you have a smooth mixture.

Gradually add the flour and salt to the mixture. Knead the dough until it forms a soft, slightly sticky ball. This may take around 5-7 minutes.

Cover the bowl, and let the dough rise in a warm place for 4-8 hours or overnight in the refrigerator.

Once the dough has risen, dust a clean surface with flour and roll out the dough into a rectangle about 1/4 inch thick.

Sprinkle the cinnamon and brown sugar mixture evenly over the rolled-out dough. Roll the dough tightly from one end to the other.

Use a sharp knife to cut the rolled dough into 1-2 inch thick rolls.

Place the rolls in a greased baking dish, ensuring they are close together but not touching. Let them rise for another hour.

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

While the rolls are baking, beat the cream cheese, powdered sugar, and vanilla extract in a bowl until smooth.

Once the cinnamon rolls are out of the oven, spread the frosting generously over them while they are still warm.