For the crumb topping: Whisk together the butter, granulated sugar, brown sugar, apple pie spice and salt. Add the cake flour and stir with a wooden spoon until dough-like. Set aside in the fridge to cool for 20 minutes.
When cool, use your fingers to break apart the mixture into large pea-sized crumbs. Hold in the refrigerator.
For the sauteed apples: Heat a small saucepot over medium heat. Add the butter and heat until melted. Add the apples, brown sugar, apple pie spice and salt and saute until the apples are slightly browned and al dente, about 10 minutes. Set aside to cool.
For the cake: Preheat the oven to 350 degrees F. Spray a 9-by-13-inch baking dish with cooking spray. Using 2 sheets of parchment, create an overlapping cross in the pan to cradle the cake and make it easy to remove once baked.
Prepare the cake batter according to the instructions on the box. Add the cake batter to the prepared baking dish, then "stud" in a layer of the cooled apples. Bake for about 20 minutes, then remove the baking dish and carefully spread the crumb topping into an even layer over the cake. Bake until the crumbs are golden and a cake tester comes out clean, another 10 to 20 minutes. Cool on a wire rack for 40 minutes. Once cooled, lift out of the pan using the parchment cradle. Dust with confectioners' sugar before serving.