Peel, core and slice the apples into a pie dish, and add hulled and washed blackberries and sugar. Make sponge by creaming butter and sugar, and adding eggs, flour and cornflour. Spread mixture over the fruit. Bake in a moderate oven (375°-M4) for 30 to 40 minutes. Sprinkle with alittle castor sugar, and serve hot with cream or Custard Sauce.