Preheat the oven to 180°C/Gas 4 and grease and line a small baking tray, roughly 24 cm x 18 cm x 5 cm. Using foil, create a firm barrier down the middle of the tray, to make sure the batters don’t run into each other.
As with so many good classic cakes, you need to start by creaming together the butter and sugar until they are pale and fluffy, almost white. This is easiest done in a stand mixer or with a hand whisk and may take as long as 5 minutes. Next, mix in the eggs, one at a time, ensuring that the first one is fully incorporated before adding the next. Finally, ditch the whisk and, using a large metal spoon, gently fold in the flour.
Add half the cake mixture into one half of the baking tray. Now add the red food colouring to the other half of the mixture in the bowl and combine thoroughly, then carefully spoon this pink cake mixture into the other half of the baking tray.
Bake for 30 minutes or until a skewer inserted in the centre comes out clean. Remove the cakes from the oven and allow to cool in the tin for a couple of minutes before turning out onto a wire rack and leaving to cool fully.
Meanwhile, roll out your marzipan into a perfect square, roughly 30 cm x 30 cm and about 3 mm thick, then keep it covered with a clean tea towel. Now all that’s left to do is the assembly. Cut the two coloured sections of cake in half lengthways and trim the edges to give you four neat, similar-sized strips. Spread each strip of cake with raspberry jam on all sides and sandwich them together in alternating pairs. Lay the sandwiched cakes in the centre of your marzipan rectangle and roll the whole thing tightly. Finally, sit the cake on a board so the seam is underneath and trim off any excess. Voila! Make some tea, slice a good chunk of Battenberg and sit back, smug as you like.