Preheat the oven to 350°F (175°C).
Grease a 9-inch cake pan lightly with butter.
Peel and slice the apples into thin wedges.
In a large bowl, whisk together flour, baking powder, cinnamon, and salt.
In another bowl, mix melted butter and sugar until smooth.
Add eggs, milk, and vanilla extract and whisk well.
Combine wet ingredients with dry ingredients to form a smooth batter.
Pour the batter into the prepared cake pan evenly.
Arrange sliced apples on top of the batter.
Sprinkle brown sugar over the apples.
Bake for 40 minutes or until golden brown and cooked through.
Allow the cake to cool for 15 minutes before slicing and serving.