Preheat the oven to 450°F. In a 10-inch cast iron skillet melt 2 Tbsp butter.
Toss the sliced apple with brown sugar and 1 tsp ground cinnamon and add to the skillet. Cook for around 7-10 minutes or until softened. Remove to a bowl and set aside. Wipe the skillet clean with a paper towel.
In a medium bowl whisk together the eggs, milk and vanilla. Add the flour, granulated sugar, cinnamon, salt and ground nutmeg. Mix until fully moistened. Set aside.
Melt the remaining 2 Tbsp of butter in the skillet. Swirl coating the bottom and sides. Pour the batter into the skillet and arrange the cooked apples on top leaving a 2-inch border to allow the edges to puff up. Save a few caramel apple slices for garnishing, if desired.
Bake for 18-20 minutes or until puffy and golden. Dust with powdered sugar and serve with a drizzle of maple syrup, if desired.