Prepare the Cinnamon Roll Dough

1. Preheat the oven to 425F. In a large bowl, add the flour, sugar, salt, and baking powder and whisk. Using a cheese grater, grate the cold butter and whisk it into the flour mixture. Set it aside.
2. Add the sourdough starter, egg, and milk to a small bowl and wrisk to combine. Then add it to the dry ingredients and stir the dough with a spoon until it is shaggy. Do NOT knead the dough. Turn the shaggy dough onto a lightly floured work surface and fold it over onto itself until all the flour is incorporated, but you can still see chunks of butter.  
3. Once the dough has come together, roll it out into a 6×9 inch rectangle and trifold it over onto itself – repeat this step 3 times. This folding and tri-folding process will give the dough lots of flaky layers. It will be sticky, so work gently, but quickly. 
4. Finally, making sure the work surface is floured well, roll the dough out into an 18×12-inch rectangle. If baking in a cast iron skillet, put the skillet in the oven now so it preheats.

Prepare the Filling & Assemble the Cinnamon Rolls

1. In a small bowl, whisk together the melted butter, brown sugar, cinnamon, and vanilla extract. Spread the cinnamon sugar mixture evenly over the rolled-out dough.  
2. If baking in a cast iron skillet, open the oven and add 2 tablespoons of butter to the hot skillet, then return it to the oven while you roll the dough so the butter browns.
3. Starting with the long end across from you, roll the dough toward you keeping it tight as you go. If the dough sticks to the work surface, just use a bench scraper to keep it from pulling apart.  
4. Once the dough is rolled up, use a sharp knife or a bench scraper to cut the cinnamon buns into 12 1-inch rolls.

Bake the Cinnamon Rolls & Prepare the Cream Cheese Frosting

1. Place the cinnamon rolls in the hot skillet with brown butter or a greased 9×13″ baking pan. Bake at 425F for 15-18 minutes or until the tops are barely golden brown.
2. While the cinnamon rolls are baking, prepare the frosting. Add the cream cheese to a large mixing bowl or bowl of a stand mixer with a whisk attachment and mix at medium speed for 2 minutes or until the cream cheese is smooth with no lumps. Add the vanilla extract and milk and whisk for another 2 minutes. Add the powdered sugar, and whisk for 2 more minutes until the frosting is smooth. Set the cream cheese frosting aside.
3. Once the cinnamon rolls are done, pour the frosting over them immediately and serve hot.