Preheat your oven to 325 degrees F (163 degrees C). Grease an 8-inch cake pan by lightly spraying with cooking spray, and line the bottom of the pan with parchment paper, and spray again. Set pan aside.
Pour the sea salt caramel sauce into the bottom of the pan, ensuring it covers the entire bottom evenly. Arrange the sliced apples neatly over the caramel sauce, slightly overlapping them. This will create an attractive arrangement. Place the pan in the refrigerator while you prepare the cake batter.
For the Cake Batter:
In a medium-sized bowl, whisk together the all-purpose flour, baking powder, ground cinnamon, ground nutmeg, and salt. Set this dry mixture aside.
Using a handheld or stand mixer fitted with a paddle attachment, cream together the room-temperature butter, granulated sugar, and brown sugar on medium-high speed. Mix until the mixture becomes smooth and creamy, which should take about 1-2 minutes.
Scrape down the sides and the bottom of the bowl with a rubber spatula as needed during the mixing process.
On high speed, add the room-temperature eggs to the creamed mixture one at a time, making sure to mix well after each addition. Then, add the pure vanilla extract and mix until fully combined.
Scrape down the sides and bottom of the bowl once more, ensuring everything is well incorporated.
Gradually add the dry ingredients mixture to the wet ingredients mixture while mixing on low speed. Simultaneously, pour in the room-temperature buttermilk. Continue mixing on low speed just until all ingredients are combined. Avoid over-mixing, and ensure there are no visible streaks of flour in the batter.
Retrieve the pan with the caramel apple topping from the refrigerator. Carefully pour the prepared cake batter over the topping, spreading it evenly with the back of a spoon.
Bake in the preheated oven for 55-58 minutes. The cake is done when a toothpick inserted into the center comes out mostly clean, with a few crumbs adhering to it.
Remove the cake from the oven and allow it to cool on a wire rack for 30 minutes.
After 30 minutes, invert the cake onto a cake stand or serving plate. Some of the caramel and juices from the topping may spill over the sides – this is expected and adds to the appeal. You can serve the cake warm, but it will be a bit messy. For neater slices, allow the cake to cool to room temperature before serving.