For the apples: Preheat the oven to 350 degrees F. Butter a 9-by-13-inch glass baking pan.
Mix together the apples, granulated sugar and cinnamon in a medium bowl. Set aside.
For the cake: Combine the butter and granulated sugar in a large bowl. Using a hand-held mixer, beat on medium high until light and fluffy, about 3 minutes. Beat in the eggs one at a time until fully combined, then add the all-purpose flour, almond flour, baking powder, baking soda, cinnamon, salt and buttermilk. Beat on low speed until a few streaks remain, then fold together with a rubber spatula until fully combined. (Do not overmix.)
Sprinkle the bottom of the pan with half of the apple mixture. Spread the batter over the top of the apples. Top the batter with the remaining apples, making decorative rows across the cake. Bake, rotating the pan halfway through, until a toothpick inserted in the middle of the cake comes out clean, about 1 hour. Allow the cake to cool slightly before dusting with powdered sugar and serving.
For a twist on a classic topping, beat 2 tablespoons store-bought salted caramel with 1/2 cup heavy whipping cream until soft peaks form. Serve dolloped on top.