Preheat oven to 450 degrees F.
In a large 10-12 inch cast iron skillet, arrange the sliced honey crisp apple around the bottom of the skillet. Drizzle 4 tablespoons of the melted butter over the apples and sprinkle the brown sugar over the top. Bake the apples in the preheated oven for 10 minutes.
While the apples are baking start on the batter. In a large stand mixer fitted with a whisk (or a blender), combine the flour, milk, eggs, brown sugar, vanilla, cinnamon, salt and 2 tablespoons of melted butter. Whisk on medium high speed for 1-2 minutes until the batter is smooth and no clumps of flour remain.
Carefully remove the hot skillet from the oven and pour the batter over the apples. Place the skillet back in the oven and bake for 15-20 minutes or until the pancake is puffed and lightly browned on top. While the pancake is baking do not open the oven during the first 10 minutes or you might deflate the pancake. While the pancake is baking start on the apple topping.