Prepare the Caramelized Apples. Start by preheating your oven to 350°F. Melt butter in a skillet over medium heat, add brown sugar, and stir until it forms a smooth caramel sauce. Pour this caramel into the bottom of a 9-inch cake pan, spreading evenly. Arrange apple slices in a circular pattern on top of the caramel, creating a beautiful base layer.
Mix the Dry Ingredients. In a mixing bowl, whisk together flour, baking powder, salt, and ground cinnamon. This dry mix will ensure an even, fluffy texture in the cake.
Cream Butter and Sugar. In another bowl, cream the butter and granulated sugar until light and fluffy. This step helps to create a light cake texture. Add eggs one at a time, beating well after each addition, then stir in vanilla extract.
Combine Wet and Dry Ingredients. Add the dry ingredients to the creamed butter mixture in thirds, alternating with milk. Mix until just combined to avoid overmixing, which can make the cake dense.
Assemble and Bake. Pour the batter over the apple layer in the cake pan, smoothing the top with a spatula. Bake in the preheated oven for 40-45 minutes or until a toothpick inserted into the center comes out clean.
Flip and Serve. Once the cake has slightly cooled, run a knife around the edges of the pan, place a plate over the top, and carefully flip the cake out of the pan. Let it cool before slicing, and enjoy the glorious caramel apple topping.