Combine dry ingredients. In the bowl of a stand mixer with a paddle attachment, combine the flour, sugar, and salt.

Mix the wet ingredients, except the butter. In a separate bowl whisk the egg, milk, and fed starter together until no lumps of starter remain.

Combine wet and dry ingredients. Turn the mixer on to low speed and gradually add the liquid ingredients. When the liquids absorb the flour and a rough dough starts to form.

Add the butter, one chunk at a time. Mix until all the butter is incorporated into the dough.

Knead the dough. Switch to the dough hook and knead the dough for about 5 minutes, until the dough is smooth and slightly sticky to the touch.

Let the dough rise. Place the dough in a bowl and cover with a damp cloth. Leave at room temperature to rise until doubled, about 10 hours. If your house is cooler it may take longer for the dough to double in size.

Make the filling and assemble

Prep your skillet. Brush a 9-inch cast-iron skillet with butter.

Roll out the dough. When the dough has doubled in size, turn it out onto a heavily floured counter and start rolling in a large rectangle ( about 18×12 inches) using a floured rolling pin.

Add the filling. Brush the melted butter all over the top of the dough. Sprinkle the cinnamon sugar filling onto the dough, leaving a 1-inch border on all sides. Gently press the filling into the dough.

Roll up the dough into a log starting with the long edge closest to you. You may need to use a bench scraper to help you roll the dough, as it may stick to the surface. If it does, just dust a little flour on the bench scraper. Roll it down to the top edge and pinch the edge to the dough log to seal it. Fold in the sides of the dough as you roll it up to prevent the cinnamon sugar from falling out on either side.

Cut the rolls. With your bench scraper or a sharp knife, slice the log into rolls 11⁄2 inches wide. Place each roll in the cast iron skillet, cut side up. You will have about 7-8 rolls. 

Second rise. Cover and place in a warm place for 3-4 hours, until the rolls have doubled in size.

Bake. When the rolls have doubled in size, preheat oven to 375 degrees F. Place the skillet in the oven and bake for about 25-30 minutes, or until golden brown. Set aside to cool while you make the frosting.