Start this recipe by cutting out a disk of non-stick paper to cover the base of a 24cm spring bottomed fluted flan tin. Place in the tin and put aside. Next, make the biscuit base. Crush the digestive biscuits to a fine crumb and mix in the melted butter. Press this mixture into the prepared tin, covering both the base and sides with the crumb mixture. Place in the fridge to firm up. Meanwhile, you can prepare the stewed apple filling. Melt the butter in a large pan with the brown sugar and add in the chopped apples, cinnamon and lemon zest. Cook together for a few minutes to soften and cook the apples to your liking. When ready, mix the corn flour with the water and add to the pan. Continue stirring until the water has thickened into a thick sauce. Carefully stir in the berries. Spoon the apple filling over the prepared biscuit base and even out. Cover with a layer of vanilla sponge and place the pie back into the fridge for 30 minutes to cool down and set. When ready, whip the cream to stiff peaks with the icing sugar, then carefully fold in the cold custard. Spoon the cream mixture over the vanilla sponge, then sprinkle over the grated chocolate and decorate with berries. Chill well before serving.