Heat your oven to 180°C (160°C fan).
Grease your cake tin well with butter.
Line the bottom with baking parchment.
This prevents sticking later on.
Melt 50g butter in a small saucepan.
Add the brown sugar and stir well.
Pour this mixture into your prepared tin.
Arrange apple slices neatly on top.
Make them overlap in a pretty pattern.
Beat softened butter and caster sugar together.
Mix until light and fluffy, about 3 minutes.
Add eggs one at a time while beating.
Fold in flour and baking powder gently.
Add milk and vanilla extract last.
Mix until just combined and smooth.
Pour batter carefully over the apples.
Spread it evenly with a spatula.
Bake for 40-45 minutes in the oven.
The cake should be golden brown on top.
A skewer inserted should come out clean.
Let the cake cool for 5 minutes.
Place a serving plate over the tin.
Carefully flip the whole thing over quickly.
Remove the tin and peel off parchment.
The apples should be on top now.