In a large bowl, combine the sourdough starter, milk, and sugar.
Add melted butter and the egg, mixing well.
In another bowl, whisk together flour and salt.
Gradually add the dry ingredients to the wet mixture, stirring until a dough forms.
Knead the dough on a floured surface for about 5-10 minutes.
Place the dough in a greased bowl, cover, and let it rise until doubled in size (about 1 hour).
Roll out the dough into a rectangle about 1/4 inch thick.
Spread softened butter over the dough, then sprinkle with cinnamon and brown sugar.
Roll the dough tightly from the long side, then cut into 12 equal pieces.
Place the rolls in a greased baking dish and let rise for an additional 30 minutes.
Preheat the oven to 350°F (175°C) and bake for 25 minutes until golden.