Combine cinnamon and sugar in a small bowl and set to the side.
Core apples and slice into ¼ inch slices. Place cast iron skillet over medium heat and add butter to the skillet. Once the butter is mostly melted add in apple slices, cinnamon and sugar and stir. Cook for 5-7 minutes
Combine eggs, flour, salt, milk, and vanilla in a medium bowl and whisk together until smooth.
When apples are done cooking pour flour mixture into the middle of the cast iron pan over the top of the apples.
Place cast iron pan carefully in the oven and bake for 18-20 minutes