Finely crush the biscuits in a plastic bag using a rolling pin, mix with the butter, chocolate, cinnamon and vanilla.
To precook the apples
Place the apples on a baking tray lined with baking paper. Bake for approx. 25 mins. in the centre of an oven preheated to 180°C.
Remove the apples from the oven, pour the cooking juices out of the centres and onto the tray. Spread the biscuit topping over the apples, bake for a further 15 mins. in the centre of an oven preheated to 180°C. Remove from the oven, serve with sour milk.