Prepare the apples: Peel, core and slice 4 of the apples. Toss them with the lemon juice and zest.
Prepare the cake batter: In a large bowl, whisk the eggs and sugar until pale and fluffy.
Gradually add the milk and melted butter, whisking continuously.
In a separate bowl, mix the flour and baking powder.
Add the dry ingredients to the wet ingredients in batches, mixing until just combined.
Stir in the vanilla extract.
Assemble the cake: Grease and flour a 23cm (9in) cake tin.
Arrange half of the prepared apple slices in the bottom of the tin.
Pour the cake batter over the apples.
Arrange the remaining apple slices on top of the batter in a decorative pattern.
Sprinkle with extra caster sugar.
Bake the cake: Bake in a preheated oven at 180°C (350°F) for 45-55 minutes, or until a skewer inserted into the center comes out clean.
Cool and serve: Let the cake cool in the tin for 10 minutes before turning it out onto a wire rack to cool completely.
Dust with icing sugar before serving.
