In a medium sized bowl, mix the eggs, flour, baking powder, sugar and salt. Mix with a whisk or blender until well combined.
Whisk in milk, sourdough discard, vanilla, melted butter, cinnamon, and butter. Whisk well until fully combined.
Refrigerate the batter for 30 minutes to 24 hours.
When you are ready to finish the pancake, preheat the oven to 425°.
Over medium heat, melt the 1/4 cup of butter in a 10" cast iron pan. Brush the melted butter up the sides of the pan with a pastry brush. Meanwhile, peel and slice the apples.
Cover the bottom of the pan with the sliced apples. Sprinkle the 1/2 cup of sugar and 1/2 tsp. of cinnamon over the apples.
Increase the heat to medium high. When the butter begins to bubble, pour the batter over the apples and butter. Turn off the heat, and move the pan to the oven.
Bake the pancake at 425° for fifteen minutes. Keeping the pan in the oven, reduce the heat to 375°. Bake for another fifteen minutes. Enjoy hot.