Or pear. Although a cake pan will make a delicious cake, a cast-iron skillet will give a better caramelization and crunchiness to the topping. The amount of sugar is up to you; if you serve this with ice cream, you could use the minimum amount (2 one-quarter cup measures). This cake is a nice breakfast treat, too.
8 T butter, divided, melted
1/2 to 1 c brown sugar, divided
2 or 3 apples, cored, and diced
1 c buttermilk (or yogurt mixed with milk; or scant 1 c milk and 1 T lemon juice–let sit for 10 minutes)
2 c all-purpose white or whole-wheat flour
1 t baking soda
1/4 t salt
1. Preheat the oven to 350ºF. Pour 4 T of the melted butter into a 9-inch cake pan or 9- or 10-inch cast-iron skillet and swirl around the bottom and up the sides. Sprinkle 1/4 to 1/2 c of the brown sugar evenly over the bottom. Scatter the diced apple over all this, smoothing it out. Set aside.
2. In a large bowl, whisk the remaining melted butter, buttermilk, eggs, and remaining sugar together until somewhat foamy. In a separate bowl, combine the flour, baking soda, and salt. Gradually add the flour mixture to the egg mixture and stir well to combine.
3. Spread the batter over the apples, smoothing and evening it out with a spatula. Bake for 45 to 55 minutes (start checking at the 45 minute mark) until the top of the cake is golden brown and a toothpick inserted into the center comes out clean. Let the cake cool in the pan for just 5 minutes.
4. Run a knife around the edge of the pan. Put a large serving plate on top of the cake pan and carefully flip so that the serving plate is on the bottom and the cake pan is on the top. The cake should fall out onto the plate. If any pieces of apple have stuck to the pan, remove them and use them to fill in any gaps on the top of the cake. Serve warm, on its own or with ice cream.