Preheat the oven to 425°F. Place a 12-inch (or 14-inch) cast iron pan into the oven as the oven preheats, with 2 tablespoons of butter in the pan and allow it to melt; watch that the butter doesn’t get brown.
In a large bowl, whisk together the eggs, milk, flour, vanilla and salt until smooth. (You can also use a blender or hand mixer for this if you prefer).
Remove the pan from the oven, and swirl so the butter coats the bottom of the pan. Pour in the batter, and return the pan to the preheated oven. Baked until puffed and golden brown, about 20 minutes.
Meanwhile, melt the remaining 2 tablespoons butter in a medium skillet. Add the apples and sprinkle over the brown sugar and cinnamon. Sauté until tender and glazed with the brown sugar and butter, about 8 minutes.
Remove the pancake from the oven and immediately spoon the apples and any sauce in the pan over the hot pancake. Dust the whole thing with the confectioners’ sugar, and cut into wedges. Serve right away, with the whipped cream if desired.