Preheat oven to 350°F (180°C) and grease a 9-inch (23cm) cake pan.
In a bowl, cream together the butter and sugar.
Whisk in the eggs, vanilla and milk until fully incorporated.
Add the flour, baking powder, salt, and cinnamon. Whisk until combined.
For the sauce, melt butter in a saucepan over medium heat, and stir in the brown sugar. Bring to a boil and continue to stir for 30 seconds, or until vigorously bubbling.
Pour the sauce into the greased cake pan.
Arrange the apples on top of the sauce in a circular pattern. Pour cake batter on top of apples and smooth the top of the batter.
Bake for 45-55 minutes, or until the cake is done (the top will spring back when gently poked).
