Lemon and pistachio battenberg

Method

1. Heat the oven to 180°C/fan160°C/gas 4. Grease a 20cm square cake tin. Cut a 30cm x 20cm strip of baking paper, then fold an 8cm pleat (4cm high once folded) in the centre. Fold a piece of tin foil to form a strip 8cm high, then sit this inside the pleat to form a solid divide. Line the tin with the pleated baking paper, keeping the divide in the centre so you can cook the 2 coloured sponges in the same tin at the same time.
2. In a large mixing bowl, beat the butter and 175g caster sugar with an electric hand mixer until light and fluffy. Beat in the eggs, one by one, until incorporated. Fold in the flour, then divide the mixture equally between 2 mixing bowls. Add the ground pistachios and green food colouring (if using) to one bowl, stirring to combine. Add the lemon zest and 40g ground almonds to the other bowl, stirring to form a smooth batter. Spoon each batter into separate sides of the prepared tin, then smooth the surfaces. Bake for 30-35 minutes until a skewer pushed into the middle comes out clean. Remove from the oven and allow to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
3. While the sponges are in the oven, make the marzipan. Combine the 225g ground almonds and the sugars in a mixing bowl. Mix the remaining ingredients together in a jug, then gradually add to the almond mixture, stirring until everything comes together into a soft, kneadable dough (you may not need all the liquid). Knead briefly in the bowl, then wrap in cling film and chill for at least 30 minutes.
4. To assemble the cake, place one cooled sponge on top of the other on a board, then trim off the crusty edges with a serrated knife so both sponges are the same size and have completely flat sides. Cut them in half lengthways to make 4 long rectangles. Now roll out the chilled marzipan on a surface well dusted with icing sugar, to a rectangle about 42cm x 22cm. Spread a layer of lemon curd over the marzipan, then place a lemon sponge rectangle on one side of an imaginary central line. Spread the inner and top side of this with lemon curd, then put a pistachio sponge rectangle next to it, so the lemon curd side is touching it. Spread the top of the pistachio sponge rectangle with lemon curd, then repeat with the other 2 rectangles on top, to form a chequer pattern.
5. Carefully bring the marzipan up and over the cake to enclose, pressing out any air bubbles as you go. Turn the battenberg over so it’s sitting on the seamed side, then dust off any icing sugar. Trim off both ends to form neat edges, then slice and serve.