Preheat the oven to 180°C (350°F).
Peel the apples and cut into thin slices. Set aside.
Melt the butter in the cake pan and add the brown sugar, stirring until smooth and bubbly.
Spread the caramel evenly at the bottom of the pan.
Arrange the apple slices on top and sprinkle with cinnamon.
In a bowl, combine the flour, baking powder, salt, and granulated sugar.
In another -smaller- bowl, beat the egg with the sugar, add the milk, the vegetable oil, and the vanilla extract.
Pour the wet ingredients into the dry and mix until just combined.
Pour the batter over the apples and smooth the top gently.
Bake for 40–45 minutes, or until golden and a toothpick comes out clean.
Let the cake cool for 10 minutes, then run a knife around the edges and flip onto a serving plate.