Prepare the Dough:
In a small bowl, combine warm milk, sugar, and yeast. Let sit for about 5 minutes until the yeast becomes frothy. In a separate large bowl, combine flour, salt, and cinnamon. Add the yeast mixture, melted butter, and eggs to the dry ingredients and mix to form a dough. Knead for about 8 minutes until the dough becomes smooth and elastic. Place it in a lightly oiled bowl, cover with a clean towel, and let rise for 1 hour or until doubled in size.

Make the Filling:
Mix softened butter, brown sugar, cinnamon, and optional nutmeg for the filling. Set aside.

Shape the Rolls:
Once the dough has risen, punch it down and roll it out on a floured surface into a rectangle. Spread the cinnamon filling over the dough, leaving a small border around the edges. Roll the dough tightly and cut it into 12 even pieces.

Prepare the Cast Iron Skillet:
Grease a 10-inch cast iron skillet with butter or non-stick spray. Arrange the cinnamon rolls in the skillet, ensuring they are snug but not crowded. Cover with a towel and let the rolls rise for another 30 minutes.

Bake:
Preheat the oven to 375°F (190°C). Bake the cinnamon rolls for 20-25 minutes or until golden brown.

Prepare the Glaze:
Whisk together powdered sugar, milk, and vanilla to make the glaze. Drizzle over the warm cinnamon rolls once they’re out of the oven.