In a saucepan, heat the topping ingredients until bubbly. Spread this mixture evenly in a 9-inch round cake pan.
Place the sliced apples on top of the sugar mixture in the pan, arranging them neatly to cover the entire surface.
In a medium bowl, whisk together flour, baking powder, and salt.
In a large bowl, beat softened butter with brown sugar and granulated sugar until light and fluffy. Add the eggs, one at a time, fully incorporating after each addition. Stir in vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with milk, starting and ending with the dry ingredients. Mix until just combined.
Carefully pour the cake batter over the arranged apple slices in the cake pan, spreading it evenly.
Bake in a preheated oven at 350°F for about 40 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool for 10 minutes, then invert onto a serving plate.