Preheat oven to 350°F (177°C).

Combine butter and brown sugar in a small saucepan over medium heat. Whisk until melted and cook for 1 minute to thicken. Remove from heat and whisk in the cinnamon and vanilla extract. Pour into an ungreased 9×2 inch pie dish or round cake pan. Arrange apple slices on top and refrigerate while preparing the cake batter.

Whisk together flour, baking powder, cinnamon, nutmeg, and salt. Set aside.

Beat softened butter on high speed until creamy. Add both sugars and continue to beat until creamed together. Add eggs and vanilla extract, mixing until combined.

Gradually add dry ingredients to wet ingredients, mixing on low speed. Slowly pour in milk while mixing on low speed until combined. Do not over-mix; whisk by hand if necessary.

Remove topping from refrigerator and pour cake batter over it.

Bake for 40–46 minutes, tenting foil halfway through to prevent over-browning. The cake is ready when a toothpick inserted into the center comes out mostly clean.

Cool on a wire rack for 15 minutes. Invert onto a cake stand or serving plate, allowing some juices to seep over the sides. Best served at room temperature.