Preheat the oven to 450 degrees F.
Arrange 3/4 of the apple slices in the bottom of a 10-12 inch cast iron skillet and drizzle 4 tablespoons butter over the apples. Sprinkle the brown sugar over top of everything. Place the skillet in the center of the oven for 10 minutes.
Meanwhile, in a blender, combine the eggs, milk, flour, vanilla, cinnamon, salt and the remaining 2 tablespoons melted butter. Blend on high for 30 seconds to one minute or until the batter is smooth. Make sure no large clumps of flour remain. Remove the hot skillet from the oven and pour the batter into the skillet. Place the skillet in the center of the oven and bake for 18-20 minutes or until the pancake is fully puffed and browned on top. Do not open the oven during the first 15 minutes of cooking or you might deflate your pancake.
Remove the Dutch baby from the oven and serve topped with the remaining apple slices, whipped cream, and cinnamon syrup.
In a small saucepan, combine the maple, butter, and cinnamon and bring to a boil over high heat. Reduce the heat to medium and simmer for 3-5 minutes. Remove from the heat and stir in the vanilla. Serve warm aside the dutch baby.