Preheat the oven to 180°C (350°F). Grease and flour a 20 cm square baking tin.
Peel, core and chop the apples up into small chunks. Place the apple juice and apples in a saucepan and simmer gently until the apple breaks up easily. Puree the apples, measure out 225 g and set aside to cool.
Beat the oil, sugar and egg in a large bowl until pale in colour and fluffy and set aside.
Sift the flour with the cocoa powder, cinnamon and bicarbonate of soda in another bowl. Stir alternate spoonful’s of the flour mixture and the apple puree into the egg mixture.
Pour the mixture into the prepared tin and place in the oven and bake for 35 minutes, or until a fine skewer inserted into the cake comes out clean.
Turn onto a wire rack and leave to cool.
Melt the chocolate for the icing in a bowl set above hot water in a saucepan over a low heat. Remove from the heat and stir in the butter making sure there are no lumps left. Spread over the top of the cake to decorate.