Keeping the apples whole and unpeeled, remove the cores with a slightly curved peeling knife, but do not make a hole right through (leave a short 'plug' at the base of the apple). Mix the cinnamon and sugar together and use the mixture to fill the apples where the core has been removed. Place a knob of butter on top of each apple then put about ½" (1cm) water in an ovenproof dish and place the apples in the dish. Bake in a preheated moderately hot oven (Mk 6 - 400ºF - 200ºC) for about 35 minutes until the apples are soft to the touch. Once or twice during cooking, baste the apples with the juices in the dish. Serve hot.