In a large mixing bowl, combine the sourdough starter, warm milk, sugar, melted butter, and egg. Mix well.
In a separate bowl, combine the flour, salt, baking powder, and baking soda. Add the dry ingredients to the sourdough mixture and stir until a dough forms.
Knead the dough for 5-10 minutes until it becomes smooth and elastic. Place the dough in a greased bowl, cover with plastic wrap, and let it rise for 1-2 hours or until it has doubled in size.
Roll out the dough into a 14”x20” rectangle on a floured surface.
In a small bowl, mix together the brown sugar and cinnamon for the filling. Spread the softened butter evenly over the dough, then sprinkle the cinnamon sugar mixture on top.
Roll the dough tightly into a log and cut it into 12 equal pieces. Place the pieces in a greased baking dish, leaving some space in between each roll.
Let the rolls rise for another 30-60 minutes or until they have doubled in size.
You can either place the rolls in the fridge to bake the next day, or you can bake right away.
Preheat your oven to 350°F (180°C). Bake in the preheated oven for 25-30 minutes or until golden brown.
While the rolls are baking, make the glaze by combining the powdered sugar, salt, melted butter, vanilla extract, and milk or heavy whipping cream. Adjust the glaze according to your preference. If you like a thicker glaze, add more powdered sugar. If you like a thinner runnier glaze, add more milk or heavy whipping cream.
Remove the rolls from the oven, pour the glaze on top, and enjoy!