Preheat oven to 180C.

Beat the butter and caster sugar in a bowl until light and creamy. Add vanilla extract.

Add the eggs one at a time, beating well.

Gently fold in the flour alternatively with the milk until you have a thick and smooth batter.

Arrange rhubarb and apple loosely in a buttered 23cm square baking dish.

Mix the extra castor sugar with the cinnamon well and and sprinkle over the fruit in the baking dish.

Spoon the batter mixture on top of the fruit and bake for 50-55 minutes, until the top is golden and cooked through. Place some foil on the rack above the baking dish so the sponge does not get burnt. 

Serve with cream or icecream.