If possible try to buy bright-green blanched and peeled pistachios to create the delicious marzipan. Alternatively, peel and blanch your own – see how in the tip below.
Heat the oven to 180°C/160°C fan/Gas 4.
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 sponge. Place the margarine, sugar, eggs, flour, semolina, baking powder and milk in a medium mixing bowl and, using an electric hand whisk, whisk together until just smooth. Place precisely half the mixture into a separate bowl.
Stir orange zest into one bowl of mixture, adding enough orange food colouring to give a good orange colour. Add the mixed spice, ground ginger and stem ginger into the other portion of mixture.
Transfer each mixture into the divided tin (one in each half), then bake for 25–30 minutes, or until risen and just firm to the touch and a skewer inserted into the centres comes out clean. Leave to cool for 5 minutes, then run a knife around the edges and turn out the sponges onto a wire rack to cool.
Make the buttercream. Place the butter, icing sugar and cocoa powder in a small bowl. Add the milk and mix with an electric hand whisk until smooth. Cover and set aside.
Make the marzipan. Place the pistachios in a food processor with the caster and icing sugars and blitz to a fine powder. Add half the egg white and all the pistachio essence and blitz for a couple of seconds to combine. TIP: To remove the skins from the pistachios, place the shelled pistachios in a bowl and cover with boiling water. Leave for 2 minutes, then drain through a sieve. Run cold water through them and drain well again. Place the nuts in a clean tea towel and rub vigorously to remove the skins. Pick out the clean nuts – some skins will remain and will need peeling with your fingertips. Spread the nuts on a baking tray. Heat the oven to 120°C/100°C fan/Gas ½. Dry out the nuts in the oven for 10 minutes, making sure they do not brown.
Tip out the marzipan and knead together, adding enough of the remaining egg white to form a soft but not sticky paste. Wrap in cling film and set aside.
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 the work surface with icing sugar and roll out the marzipan to a 23 x 32cm rectangle. Trim the edges to neaten. Spread the marzipan thinly with buttercream. Take one of the long rectangles of orange sponge and one of the ginger sponges and place them side by side in the centre of the marzipan. Spread a little buttercream between the strips to help them stick.
Spread buttercream on top of the sponges, then place 2 more strips of sponge on top, arranging them to give a chequerboard effect and spreading buttercream between them to stick them together.
Fold the marzipan neatly over the sponges to enclose them. Trim and smooth the join if necessary and trim the ends to neaten.
To decorate, mix the icing sugar with just a tiny drop of water to make a thick pouring consistency and add a little orange colouring. Place the icing in the piping bag, snip the end and drizzle icing over the cake. Decorate with strips of stem ginger and candied orange peel arranged in crosses down the centre of the cake.