This green cake was artist Claude Monet’s favourite birthday cake.
Make the marzipan. Grind the pistachios in a food processor. Add the icing sugar and grind until fine, then tip into a mixing bowl. Add the egg white and pistachio essence and combine. Turn out onto a worktop dusted with icing sugar and knead until smooth. Wrap in cling film to rest.
Heat the oven to 180°C/160°C fan/350°F/Gas 4. Make the génoise. Finely grind the pistachios and flour in a food processor. Place the sugar and eggs in a large bowl and whisk until thick and pale. Fold in the flour mixture, the butter and the lemon zest.
Pour the mixture into a cake tin and bake for 20–30 minutes, until risen and springy to the touch. Set aside to cool for 5–10 minutes, then turn out onto a wire rack to cool completely.
Make the crème au beurre. Bring 125ml of water to the boil in a large pan. Add the spinach and wilt for 2–3 minutes. Transfer to a blender and blitz to a purée.
Drain the purée through a piece of muslin, squeezing out all the spinach water into a bowl. You should have 160–170ml. Set aside.
Grind the pistachios in a blender. Add the kirsch, 1 tablespoon of the spinach water and 50g of the butter. Blitz to a paste and set aside.
Put the sugar and 100ml of the spinach water into a pan (reserve the remainder). Dissolve the sugar gently, then boil for 2–3 minutes, until the syrup is clear and forms a thread when pulled between 2 teaspoons.
Whisk the egg yolks in a stand mixer, then pour in the syrup in a thin stream, whisking all the time. Whisk until the mixture is thick and cold, then add the remaining 100g of softened butter, and whisk in the pistachio paste.
Slice the sponge into three equal layers. Spread the bottom layer with one third of the crème au beurre, then the middle layer with another one third, then top with the final sponge. Spread a thin buttercream layer over the top and sides. Roll out the marzipan and use to cover the whole cake.
To make the decoration, sift the fondant icing into a bowl and, a little at a time, add 3–4 tablespoons of the remaining spinach water until you have a thick, pourable fondant. Add the pistachio essence (if using) and mix thoroughly until completely smooth.
Stand the cake on a wire rack and pour over the fondant icing. Leave to drip for 2–3 minutes, then transfer to a cake stand. Press the chopped pistachios around the base of the cake, then decorate with edible flowers.