1. You must prepare the topping first by placing the demarara sugar and 3 tablespoons of water in a small saucepan and heating gently until the sugar has dissolved. Stir with a wooden spoon to ensure it has dissolved, and bring to the boil. Boil rapidly until golden caramel has formed. Pour into the base of a 7" (18cm) deep cake tin and leave to cool for 2 or 3 minutes. 2. Carefully swirl the caramel around and up the sides of the tin until lightly coated. Set aside. 3. Place an upturned saucer in the slow cookers cooking dish and pour in 1" (2.5cm) or cold water. Switch slow cooker to high. 4. Cream together the butter and sugar until light and fluffy. Beat in the eggs a little at a time, beating after each addition. Adding a spoonful of flour after each addition helps to prevent curdling. 5. Melt the chocolate and then stir this into the mixture. 6. Sift in the rest of the flour and cocoa powder, and add 1 to 2 tablespoons of cooled boiled water. Mix well to give a soft dropping consistency. 7. Remove the peel from both oranges - try and get as much of the pith off as possible also. Thinly slice the peel into strips and then slice the oranges. 8. Arrange the peel then the orange slices on the caramel. Top with the chocolate mixture and smooth the top. Cover with a pleated sheet of lightly oiled tin foil. 9. Place the tin in the cooking dish and pour around sufficient boiling water for it to be halfway up the side of the tin. Cover with a lid and cook for 4 to 6 hours, until the top feels firm. 10. Remove from the slow cooker and leave for 5 minutes before serving. Sprinkle a little more cocoa powder on top if you wish.