Preheat the oven to 180˚C/160˚C fan/gas mark 4. Grease and line a deep 20cm (8inch) square cake tin. In a mixing bowl, beat together the butter, brown sugar and Vanilla Extract until well blended.
Sift the flour and Baking Powder on top and stir together to form a crumbly mix. Press half the mixture into the base of the prepared tin to make a smooth base. Prick the base all over with a fork and bake for 20 minutes until lightly golden and just firm.
Whilst the base is baking, peel and core the apples, then chop into small chunks and place in a large bowl. Add the blackberries and pour the lemon juice into the bowl. Toss the fruit in the lemon juice – this prevents the apples from browning. Sprinkle the corn flour and cinnamon over the fruit and mix through.
Once the base has baked, spoon the fruit mixture over the base and spread out evenly. Sprinkle the remaining crumble on top of the apple, making sure it retains the crumbly texture. Bake for 35-40 minutes until golden brown and firm to the touch. Leave to cool for 10 minutes then cut into 12 chunky squares. Leave in the tin to cool completely before carefully removing and placing on a board.
To serve, dust lightly with icing sugar. Your cake is now ready to serve and enjoy!