Preheat Oven: Preheat your oven to 350°F (177°C).
Prepare Topping: Melt butter and brown sugar in a saucepan over medium heat, whisking until thickened (about 1 minute). Remove from heat, stir in cinnamon and vanilla, and pour into a 9-inch round cake pan. Arrange apple slices on top and refrigerate.
Mix Dry Ingredients: Whisk flour, baking powder, cinnamon, nutmeg, and salt together in a bowl.
Cream Wet Ingredients: Beat butter until creamy. Add sugars, then eggs and vanilla, beating until combined.
Combine Ingredients: Gradually mix dry ingredients into wet, alternating with milk, until the batter is smooth. Avoid over-mixing.
Assemble Cake: Pour the batter evenly over the chilled apple topping.
Bake: Bake for 40–46 minutes, covering with foil halfway through. Check doneness with a toothpick—it should come out with a few moist crumbs.
Cool and Serve: Cool the cake for 15 minutes, then invert onto a serving plate. Let it cool completely before slicing for neater pieces.