Preheat oven to 350°F (175°C). Grease and flour a 9×13-inch baking dish or a 9-inch round springform pan.

1. Peel, core, and slice the apples thinly. Set aside.
2. In a medium bowl, whisk together flour, baking powder, cinnamon, nutmeg (if using), and salt.
3. In a large bowl, beat the butter with granulated and brown sugar until light and fluffy.
4. Add eggs one at a time, beating well after each addition. Stir in the vanilla extract.
5. Alternate adding the dry ingredients and milk (or sour cream) to the butter mixture, beginning and ending with the dry. Mix until just combined.
6. Spread half of the batter in the prepared pan. Layer with half of the apple slices.
7. Spread the remaining batter over the apples and top with the remaining apple slices in an even layer.
8. Sprinkle with a little cinnamon sugar, if desired.
9. Bake for 45–50 minutes, or until a toothpick inserted into the center comes out clean.
10. Let cool before dusting with powdered sugar or serving with whipped cream.