In a large mixing bowl add the brown sugar, oil, egg, buttermilk, and vanilla. Mix the baking soda, flour, cardamom and nutmeg together and add to the mixing bowl. Whisk until the ingredients come together. Note that leaving it a little lumpy will make it fluffier and less dense, so combine well enough to not have threads of white flour, just don't overmix.
Gently fold in the apples with a baking spatula until well and evenly coated, scraping the sides of the bowl as needed. Pour into the baking dish.
Cinnamon Sugar Topping:
In a medium bowl, add the sugar and cinnamon and stir to mix. Add the melted butter and using a fork, combine the ingredients until it resembles coarse sand. Add the topping to the batter as evenly as possible, using the fork or the back of a spoon to help spread it out.
Bake 40 minutes to 1 hour - this will depend on the apples and the moisture in the batter, so check at the 40 minute mark and use your judgment. To test for doneness stick a toothpick, or a knife, into the center of the batter - it should come out with a scant amount of batter on it - it shouldn't come out entirely clean or overly wet for best results.
Let it cool a bit before serving, about 30 minutes or longer. Serve warm or room temperature.
Notes
Things to know before you start:I've made this with only one apple and it was still wonderful, so don't get caught up on the size of the apples. I've also substituted buttermilk with sour cream and while it was a thicker batter to mix, it still baked up incredibly tender and delicious. The flavor profile of the cake is more subtle but worked beautifully.How to properly measure flour:Fluff the flour in the container with a spoon to aerate it. Then, gently spoon the flour into the measuring cup until it's overflowing, taking care not to tap or pack it. Gently sweep off excess flour with the back of a knife. Does measuring flour the right way matter? Yes. Because scooping flour straight from its bag or canister packs the flour and ultimately adds more flour than is needed to the recipe.