First make the jelly. Line a 20cm (ish) shallow bowl with cling film.Remove the zest from 4 of the oranges and set aside for the cake. Tip the orange juice and 100G of sugar into a saucepan and gently heat to dissolve the sugar. Meanwhile, soak the gelatine leaves in cold water for a few mins until soft. Remove and squeeze out any excess water and add to the warm orange juice, stir until the gelatine has dissolved. Pour the liquid into the lined bowl and chill for at least 2 hrs.
Heat oven to 140C fan/gas 3. Tip all the cake ingredients into a large mixing bowl and combine with an electric hand whisk until smooth. Spoon into a lined frying pan (you can use a lined cake tin but the pan gives a more authentic "jaffa cake" look) and smooth over the surface. Bake in the centre of the oven for 55 mins, or until a skewer inserted comes out clean. remove and cool
Now make the ganache. Heat the cream in a small pan until hot. Put the chocolate in a small bowl and pour over the cream, leave for 10 mins, then mix well – you should be left with a smooth chocolate sauce. Leave at room temperature until the ganache cools and thickens a little (you can put it in the fridge to speed this up, but keep an eye on it, as it will set quickly).
To assemble the cake, place it on a cake stand or chopping board and trim the top to give you a flat surface, turn the jelly out of the bowl and place on top rounded side up. Then gently pour over the chocolate orange ganache and using a spatula work around the jelly until completely covered and to finish the look use a skewer to make a kriss cross effect on top.