Melt the butter in a 10-inch cast iron skillet over medium heat. Add the apples and saute until they begin to soften, about 4 minutes. Sprinkle the cinnamon over them and stir to combine. Distribute the apples evenly in the skillet.
Preheat the oven to 500 degrees F. Whisk the eggs, flour, milk, vanilla and salt to make a smooth batter.
Pour the batter over the apples in the skillet. Bake the pancake until puffed up and set in the center, 10-12 minutes. Divide into triangles and serve with your desired toppings.