This lemon and white chocolate cake is a single-tier version of Hermine’s creation in Episode 4. Strips of chocolate form a layered collar around the outside, concealing the layers of delicate sponge within.
Heat the oven to 180°C/160°C fan/350°F/Gas 4.
Make the sponges. Melt the white chocolate in a bowl over a pan of simmering water. Stir until smooth, then remove from the heat.
Place the caster sugar, eggs and vanilla extract in the bowl of a stand mixer fitted with the whisk and whisk on medium speed for 2–3 minutes, until pale, at least doubled in volume and the mixture leaves a ribbon trail when you lift the whisk.
Sift the flour, baking powder and salt into the bowl and fold in using a large metal spoon. Add the melted chocolate and fold again until thoroughly combined.
Divide the mixture equally between the prepared tins and bake for 20 minutes, until golden, well risen and a skewer inserted into the centres comes out clean. Remove from the oven, leave to cool in the tins for 2 minutes, then turn out onto a wire rack to cool completely.
Make the lemon curd. Whisk the eggs, egg yolks and sugar in a small pan. Add the lemon juice and grated zest and whisk to combine.
Add the cubed butter, then place the pan on a low heat and, whisking continuously, cook until the butter has melted and the curd thickens to coat the back of a spoon. Strain the curd into a clean bowl, cover the surface with cling film and leave to cool. Then, chill for at least 1 hour, or until ready to use.
To make the Italian meringue buttercream, combine the sugar and 4 tablespoons of water in a small pan set on a low–medium heat. Leave the sugar to dissolve, then bring the syrup up to the boil and cook until it reaches 118°C/244°F on the sugar thermometer.
Meanwhile, whisk the egg whites and pinch of salt in the bowl of a stand mixer fitted with the whisk attachment until they hold soft peaks.
Continue whisking and as you do so, slowly pour the hot syrup onto the egg whites. Continue to whisk on medium speed until the meringue is completely cold and it holds stiff, glossy peaks.
Add the butter a few cubes at a time to the cold meringue, whisking continuously until smooth. Add the vanilla extract and mix until combined. Spoon half the buttercream into the piping bag and set both portions aside.
Prepare the ganache. Heat the cream in a small pan until just boiling. Place the chocolate in a heatproof bowl and pour over the hot cream. Stir gently until smooth, then leave to cool to room temperature and until spreadable.
To assemble the cake, place one sponge layer on a serving plate and spread with half the lemon curd. Pipe over an even layer of buttercream and top with the second sponge, gently pressing it in place. Top with another layer of lemon curd and buttercream and finish with the final sponge. Using a palette knife, spread the remaining buttercream around the outside of the cake.
Spread the ganache over the top of the cake and then chill the cake while you prepare the white chocolate collar.
Melt 200g of the white chocolate in a bowl set over a pan of gently simmering water. Stir until smooth, then remove from the heat and leave to cool slightly.
Lay the acetate strips on your work surface and spread them evenly with the melted white chocolate. Transfer the strips onto the lined baking tray and place in the fridge to chill for about 5 minutes, until the chocolate is just set.
Wrap 2 strips around the outside of the cake along the bottom edge, then chill the cake until set firm and carefully peel away the acetate. Repeat three more times, until you have covered the sides of the cake in 8 strips of white chocolate.
Melt the remaining chocolate as in Step 15. Spread it onto a clean baking sheet and place it in the fridge for about 15 minutes, until set. Holding the blunt end of the blade of long knife, drag the sharp edge across the chocolate to create curls. Arrange these on top of the cake to finish.