Heat the oven to 425 F. Put a cast iron or oven-proof 9- or 10-inch skillet in the oven.
In a bowl, combine the apple slices with brown sugar and cinnamon; toss gently to coat well.
In another bowl, mix flour with salt. In a large cup or small bowl, whisk together the eggs, milk, vanilla, and 1 tablespoon of the melted butter.
Carefully take the hot cast iron pan from the oven, set it on a cooling rack, and add remaining butter; swirl to coat the bottom of the pan.
Arrange apple slices over the bottom of the pan.
Pour batter over apple slices.
Bake for 25 to 30 minutes, until puffed and lightly browned.
Serve with syrup or sprinkle with powdered sugar.