Preheat oven to 425 degrees.
Spray a 10-inch cast iron skillet with cooking spray, and wipe it out.
Place the skillet in the oven to preheat.
In a large bowl, whisk together flour salt, sugar, and cinnamon. Set aside.
In a medium bowl, combine eggs, milk, and vanilla.
Stir wet ingredients into dry ingredients. Set aside for 20 minutes.
While batter is resting, peel, core, and slice apple.
Toss apple with salt, cinnamon, and sugar. Set aside.
When the 20 minute rest period is over, add butter to cast iron skillet in oven.
When butter melts, remove from oven and pour batter into skillet on top of butter.
Return to oven for 10 minutes.
Reduce heat to 350 degrees and cook for another 8-10 minutes, until puffy on the bottom.
While the pancake is cooking, sauté apple mixture in 2 tablespoons salted butter until apples are tender, 2-3 minutes. Set aside.
When pancake comes out of the oven, top with apple mixture and serve.