Prepare the Dough: In a large mixing bowl, combine the sourdough starter, flour, sugar, salt, melted butter, milk, egg, and vanilla extract. Mix until the dough begins to come together. Knead for about 5-7 minutes until smooth and elastic.

Let It Rise: Cover the dough with a damp cloth and let it rise in a warm area for 4-6 hours or until doubled in size.

Prepare the Filling: While the dough rises, combine the brown sugar and cinnamon in a small bowl. Set aside.

Roll Out the Dough: Once the dough has risen, punch it down and transfer it to a floured surface. Roll it out into a large rectangle (about 12×18 inches).

Spread the Butter and Add the Filling: Spread the softened butter evenly over the dough. Sprinkle the cinnamon-sugar mixture over the buttered dough.

Form the Rolls: Starting from one long edge, carefully roll the dough tightly into a log. Slice it into 9-12 rolls, depending on your preferred size.

Arrange in Cast Iron Skillet: Place the rolls in a greased 10-inch cast iron skillet. Cover with plastic wrap and let the rolls rise for about 1 hour.

Preheat the Oven: Preheat the oven to 350°F (175°C).

Bake the Rolls: Bake for 25-30 minutes, or until golden brown. Brush the tops of the rolls with a tablespoon of milk to give them a nice sheen.

Serve: Let the rolls cool for 10 minutes before serving. Dust with powdered sugar if desired.