Preheat the Oven: Preheat the oven to 350°F (177°C), preparing it for baking while you assemble the topping and cake batter.
Prepare the Caramel Topping: Place the butter and brown sugar in a small saucepan over medium heat. Whisk occasionally until the butter melts, then continue cooking while whisking constantly for 1 minute, until the mixture thickens. Remove the saucepan from the heat and whisk in the ground cinnamon and vanilla extract. Arrange the Apples: Pour the caramel mixture into an ungreased 9-inch pie dish or round cake pan that is at least 2 inches deep. Peel the apples and cut them into ¼-inch-thick slices. Arrange the slices over the caramel, overlapping them as needed to cover the surface. Refrigerate the pan for a few minutes to help the topping set while you prepare the batter. Combine the Dry Ingredients: In a mixing bowl, whisk together the flour, baking powder, ground cinnamon, nutmeg, and salt until evenly combined. Set aside.
Cream the Butter and Sugars: Using a handheld electric mixer or stand mixer fitted with a paddle or whisk attachment, beat the softened butter on high speed for about 1 minute, until smooth and creamy. Add the granulated sugar and brown sugar, then beat on high speed for another minute until the mixture is well creamed. Scrape down the sides and bottom of the bowl with a rubber spatula as needed. Add the Eggs and Vanilla: Add the eggs and vanilla extract to the butter-sugar mixture. Beat on high speed until the ingredients are combined, scraping down the bowl as necessary to incorporate everything evenly.
Finish the Cake Batter: Set the mixer to low speed and gradually add the prepared dry ingredients while slowly pouring in the room-temperature milk. Mix only until the ingredients are incorporated. Avoid overmixing. If needed, gently whisk the batter by hand to remove any lumps remaining at the bottom of the bowl. The finished batter should be slightly thick. Assemble the Cake: Remove the chilled topping from the refrigerator. Spoon or pour the cake batter over the arranged apples and spread it evenly across the pan, covering the topping completely.
Bake the Cake: Place the pan in the preheated oven and bake for 40–46 minutes. Halfway through baking, loosely tent the top with aluminum foil to prevent excessive browning before the center finishes cooking. The cake is ready when a toothpick inserted into the center comes out mostly clean, with a few moist crumbs. Baking may take longer in some ovens, and the cake may rise enough to touch the foil. Cool Briefly and Invert: Remove the cake from the oven and place the pan on a wire cooling rack for 15 minutes. Carefully invert the slightly cooled cake onto a cake stand or serving plate. Some caramel juices may run over the edges; this is normal for an upside-down cake. Cool and Serve: The cake can be served warm, although the slices may be messy. For neater slices and a better-set texture, allow it to cool completely at room temperature before cutting. Do not refrigerate the cake to speed up cooling, as this may make its texture overly dense. Store Leftovers: Cover any leftover slices and refrigerate them for up to 3 days or freeze them for up to 3 months. Thaw frozen slices at room temperature before serving. Freezing the entire cake is not recommended because the apple topping may not retain its arrangement after thawing.
