Preheat the oven to 180 C, Gas 4. Peel and core the apples then cut them in half. Cut the halves into 5 mm slices. Sprinkle them with a little lemon juice.
Whisk the eggs with the 100g of sugar and the vanilla in a bowl until thick and creamy . Sift in the flour and baking powder and mix to incorporate.
Add the milk and the olive oil and stir to make a smooth batter.
Butter and flour a 24cm cake tin and pour in the batter which will spread to cover the bottom. Pat dry the apple slices and arrange them on top of the batter.
Sprinkle with the remaining sugar and dot with the butter.
Bake for about 45 minutes or until the top is golden. The cake inside should be moist and spongy.