Preheat your oven to 350°F. Grease a 9-inch x 12-inch (or 9-inch square) baking dish.

On the bottom of the baking dish, scatter the chopped apples in an even layer in the bottom. Add the ground cinnamon, grated nutmeg, and 2 tablespoons of the cake mix, and toss to coat. Spread the apples into an even layer, and set the dish aside.

Place the remaining cake mix in a large bowl.

Using a standard grater, grate the chilled butter into the cake mix and, using your clean hands, toss to coat the butter in the cake mix.

Break up the lumps that form in the grated butter until the mixture is uniform and looks like very coarse sand. Scatter the butter and cake mix mixture in an even layer on top of the chopped apples.

Pour the chilled buttermilk evenly over the top.

Sprinkle the top of the cake very lightly with ground cinnamon.

Cover the baking dish tightly with aluminum foil, place it in the center of the preheated oven and bake for 25 minutes.

Remove the foil, return the cake the oven and bake until the apples are tender and the top is puffed and lightly golden brown (about another 15 minutes for the 9-inch x 12-inch pan, 20+ minutes minutes for the 9-inch square pan).

Remove from the oven and allow to cool in the pan for 10 minutes before serving with a dollop of whipped cream and a very light sprinkling of ground cinnamon.