Ingredients (makes 8)
For the Biscuit Sponge
- 4 eggs
- 100g ground pistachios
- 50g ground almonds, grind your own in a food processorif necessary
- 150g icing sugar
- 45g flour
- 5 egg whites
- 25g sugar
- 40g unsalted butter, melted
For the Ganache
- 100ml milk
- 4 cardamom pods, bruised
- 200g dark chocolate, roughly chopped
- 20g unsalted butter, softened
For the Soaking Syrup
- 60g sugar
- 60ml water
- 4 cardamom pods, bruised
- 100ml orange juice
For the Orange Buttercream
- 75g unsalted butter, softened
- Zest from 1 large orange
- 150g icing sugar
- 1 tsp fresh orange juice
- 1 quantity Mirror Glaze (recipe here)
- 2 x 20x30cm/8x12-inch Swiss roll tins, greased and fully lined with baking paper
20cm/8-inch square cake board (optional but handy)
Preheat the oven to 200°C/180°C fan/Gas 6.
Place the eggs, ground pistachios, ground almonds, icing sugar and flour into a mixing bowl and beat together until well combined and smooth. It is best to do this in a freestanding electric mixer with whisk attachment, or at least using an electric handheld mixer. In another bowl, whisk the egg whites to stiff peaks, then add the sugar in three incorporations, whisking well after each addition, until the sugar is dissolved. Scoop this meringue on top of the egg and nut mixture, and pour on the melted butter. Gently fold together until just incorporated, trying not to expel too much air. The mixture should be fairly runny.
Divide the mixture between the prepared tins and smooth the surfaces. Bake for 12–15 minutes, or until lightly golden in colour and when pressed lightly the sponge springs back. Remove from the oven and allow to cool completely before removing from the tins, trying not to break any sponge.
As the sponges bake, make the ganache. Place the milk in a saucepan with the bruised cardamom pods and set over a high heat. When the mixture begins to steam, remove from the heat and allow the cardamom to infuse into the milk for about 10 minutes. Remove the cardamom pods and reheat the milk on medium- high until bubbles just begin to form around the edges. Place the chopped chocolate in a heatproof bowl and pour the hot milk over it. Allow the milk to heat the chocolate for about 30 seconds, then use a whisk to blend together to a smooth, glossy ganache. Whisk in the butter until it is well incorporated, then pour on to a large plate and set aside to cool.
To make the soaking syrup, place the sugar, water and bruised cardamom pods in a saucepan and set over a high heat. Bring to the boil, allow to boil for about 3 minutes, then turn off the heat and leave to cool slightly. Stir in the orange juice and allow to infuse.
To make the buttercream, place the butter and zest in a bowl and whisk until pale and smooth, then sift in the icing sugar and add the orange juice. Beat slowly at first until the butter and icing sugar come together, then whisk vigorously for at least 5 minutes – this is where you need a freestanding mixer with whisk attachment or at least a handheld electric whisk. The buttercream should become very pale and light.
To assemble, cut the two sponges into two pieces: you need from each a square of 20x20cm, and a piece of 10x20cm (so in total two 20cm squares and two 10x20cm rectangles). Place one of the 20x20cm squares on a board and soak it well with some of the syrup using a pastry brush. Then take the ganache and test if it is ready: you should be able to spread it into a mound, which should hold its shape. Spread two-thirds of the ganache on to this first layer – don’t worry if it oozes down the side as you will trim it anyway. Chill for 20 minutes.
Place the two rectangular pieces of sponge on the ganache and soak well with some syrup, then take the buttercream and gently spread that on to the sponges, again not worrying if some dribbles down the side. Chill for 20 more minutes.
Take the final square of sponge and soak it well with syrup then place soaked-side-down on to the buttercream. Spread the remaining ganache evenly over the top of the cake, and place in the fridge, or ideally the freezer, for a good 20 minutes.
Make the mirror glaze as described in the link above, or, if you made it earlier and it has set, place it in a microwave and heat on medium for 30-second bursts until glossy and runny again. Pour this over the chilled cake, ensuring the glaze sets evenly on top – it doesn’t need to dribble down the sides, but if it does, again don’t worry. Put back into the fridge for a good 20 minutes to set.
When the mirror glaze has set, take a very sharp knife, dip it in a large jug of boiling water, then dry quickly on a cloth. Slice a tiny amount from each edge – just 5mm if you can. This will create nice, defined layers. Then, repeating the knife-heating technique, cut the opera into 4 equal squares, and cut each square in half into equal slices. You will have 8 slices. Decorate with as much – or as little, if you have the self-restraint I clearly lack – as you like of gold leaf.