Start with making the marzipan. Place the ground almonds and icing sugar in a food processor and blend until fine, then whilst the motor is still running, gently pour in the egg white a little at a time until it starts to come together.
Tip out onto a cold surface and knead to combine into a smooth ball. If it’s too wet, add some more icing sugar until it reaches a stable consistency- the marzipan should feel soft and moist to the touch. Leave to rest in the fridge for at least an hour.
Next, the sponges. Weigh all the dry ingredients together in a bowl and set aside. Then either using a food processor or a hand mixer, cream the sugar and butter together until really pale and fluffy.
Add one egg at a time with a spoonful of flour, and beat until combined. Repeat this with the next two eggs, then fold in the rest of the dry ingredients and lemon zest. Split this mixture into two and gently whisk in 3 egg yolks into one half, and the beet juice/food colouring into the other.
Pour these mixtures into two rectangular tins, around 25cm length or so. Cook on 180◦ for around 10-15 minutes. The cakes will have raised and be bouncy to the touch when cooked. Leave to cool, then pop in the fridge for 15 minutes before constructing.
For the buttercream, beat together the soft butter and icing sugar until it’s light, white, and fluffy. Add the elderflower cordial to taste- I like to use around 25ml or so, so it’s super sharp and floral.
Assembling
Take the chilled cakes, trim any wonky sides off and mark 2.5cm lengths on the cake before slicing into layers. Flip the length on its side and further trim to 2.5cm width so it’s squared off.
Now take your marzipan, knead until soft and split into two. Roll out large enough to fully cover your sponges, regularly rotating it and dusting icing sugar to stop it from sticking. When you reach around 2mm thick, you’re ready to go.
Trim the marzipan so you have a neat sheet to use, and spread the elderflower buttercream over liberally. Arrange one yellow and one pink sponge lengthways at the very edge of the marzipan, sandwiching with buttercream between to stick them together.
Alternate the other sponges and again, apply buttercream in between. Then gently roll the Battenberg on its side, adding a little pressure to stick it together. Roll it over again, press gently and neatly cut off the excess marzipan to form a square.
Chill the Battenberg in the fridge before slicing and serving with tea.