Heat oven to 180C/fan 160C/gas 4. Grease a 2-litre/3½-pint ovenproof dish lightly with butter. Tip the flour, sugar and baking powder, along with a pinch of salt, into a large bowl. Mix together the milk, butter, egg and vanilla extract and stir into the dry ingredients until you get a smooth batter. Arrange the apples in the dish, spoon the batter on top and smooth with a knife until the apples are covered.