Preheat the oven to 180°C. Grease a deep 20 x 30cm shallow slice tin with some melted butter then line the base and sides with baking paper. In a mixer, beat the butter and sugar until light and fluffy then gradually add the eggs. Stir in the flour, vanilla and buttermilk and mix until smooth. Peel, quarter and core the apples and then slice very thinly.
Pour a thin layer of the mixture into the baking tin then arrange apple slices on top. Pour over the remaining mixture and scatter berries on top. Bake for 40 mins or until golden. Allow to cool in the tray for 10 mins before dusting with icing sugar.