This moist cake gets its flavour from a whole cooked orange – make sure you use a thin-skinned orange, or use mandarins.
Heat the oven to 190°C/170°C fan/Gas 5.
Fold the 2-in-1 parchment and foil in half. Holding the folded edge, fold it over by 5cm, then open up the paper on either side and re-crease the centre folds to make a divider. Place this in the centre of the prepared cake tin.
Make the orange sponge. Place the whole orange or mandarins in a saucepan. Cover with boiling water, place a lid on top and simmer for 15 minutes, until tender.
Drain the orange or mandarins, slice them in half and discard any pips. Place in a food processor and blitz to a chunky purée. Measure out 75g of the purée and place it in the freezer to cool. Discard the remainder.
Meanwhile, place the eggs and sugar in a medium bowl and whisk with an electric hand whisk until light and foamy. Keep whisking and pour in the oil, then whisk in the orange food colouring to give an even colour.
Add the cooled purée and the ground almonds, flour and baking powder to the mixture and set aside.
Make the chocolate cake. Place the flour, sugar, cocoa powder, baking powder and salt in a medium mixing bowl. Using a balloon whisk, whisk in the egg, milk, oil and vanilla until smooth and shiny, then whisk in the boiling water to make a thin batter.
Pour the chocolate mixture into one side of the prepared cake tin and the orange mixture into the other side. Bake for 25–30 minutes, or until risen and just firm to the touch. Leave to cool in the tin for 10 minutes, then run a knife around the edges and turn out onto a wire rack to cool completely. Reduce the oven to 100°C/80°C fan/Gas ¼.
Begin the candied orange slices. Cut the oranges into thin, round slices and lay them out on a wire rack over a baking tray. Place the slices (with the rack and baking tray) in the oven for 45 minutes, until they have dried out.
Meanwhile, make the ganache. Place the double cream in a pan on a medium–low heat until just boiling. Remove from the heat, add the chocolate and stir until smooth. Transfer the ganache to a shallow dish and allow to cool, then chill until spreadable.
Make the marzipan. Place the ground almonds and icing and caster sugars in a medium bowl and add most of the egg white. Stir to combine, adding more egg white if needed to form a pliable but not sticky dough. Cut off one sixth of the dough and knead the cocoa powder into this piece. When you have an even colour, break off pieces bit by bit, rolling each into a large pea-sized ball. Cover these with cling film, wrap the large piece of marzipan in cling film, and set both aside.
Remove the orange slices from the oven and sprinkle liberally with caster sugar on both sides, shake off and excess sugar and halve into decorative slices. Set aside.
Melt 50g chocolate in a small bowl over a pan of gently simmering water, ensuring the bowl does not touch the water. Heat until it reaches 45°C on a cooking thermometer, then remove from the simmering water and add the remaining chocolate. Stir until melted and allow to cool to 28°C, then return to the simmering water for a minute or two until it reaches 32°C. Remove from the heat, then spread onto a sheet of acetate or baking paper in 5cm squares. Place in the fridge to set.
Assemble the battenberg. Level the top of each sponge and stack one on top of the other. Trim the sides so that each sponge is a neat rectangle, the same size as the other. Cut through the centre of both sponges lengthways to give 4 equal, long rectangles. Set aside.
Dust a sheet of baking paper and your rolling pin with icing sugar. Place the orange marzipan on the baking paper and roll out to almost an 18 x 30cm rectangle. Press the chocolate marzipan balls into it to make a polka-dot pattern, then roll again gently to combine without spreading the chocolate marzipan too much, until you have a full 18 x 30cm rectangle with chocolate dots. Cover with another sheet of baking paper and a baking sheet and invert, then remove the uppermost piece of baking paper.
Trim the edges of the marzipan to make a neat rectangle. Spread the marzipan thinly with ganache, then take one of the long rectangles of orange sponge and one of the chocolate sponges and place them side by side in the centre of the marzipan. Spread a little ganache between the strips to help them stick.
Spread ganache on top of the sponges, then place 2 more strips of sponge on top, arranging them to give a chequerboard effect and spreading ganache between them to stick them together.
Fold the marzipan over the sponges to enclose them. Trim and smooth the join if necessary and trim the ends to neaten.
To decorate, arrange the shards with the orange slices along the centre of the cake, using a little of the remaining ganache to help fix them in place.