Melt the butter in a 10-inch cast iron skillet over medium heat. Remove 1 tablespoon of the melted butter and set aside. To the melted butter, add the brown sugar and melt them together until the sugar is dissolved, about 2 minutes. Remove from the heat.
Place one of the pie crusts over the melted butter and brown sugar mixture in the skillet. Pour the apple pie filling over the crust and sprinkle with 1 tablespoon of the cinnamon sugar. Cover the filling with the second pie crust. Brush the top with the reserved melted butter, then evenly sprinkle with the remaining 1 tablespoon cinnamon sugar on top. Cut vents in the middle of the pie.
Bake until browned and bubbling, about 30 minutes. Serve hot with your favorite ice cream (Terry recommends butter pecan!) or cinnamon whipped cream.