A joconde sponge is a decorative almond-flavored sponge cake made in layers. Opéra gâteau is an elaborate version of it, here made with kirsch syrup and a chocolate ganache.
Equipment and preparation: You will need a 33x23cm/13x9in Swiss roll tin and a free-standing food mixer.
For the kirsch syrup, tip the sugar into a small pan. Pour over 200ml/7fl oz water and cook over a medium heat until the sugar has dissolved. Bring to the boil and simmer for 4-5 minutes, or until the syrup has reduced by half. Remove from the heat, add the kirsch, stir and leave to cool completely.
For the joconde sponge, heat the oven to 220C/425F/Fan 200C/Gas 7. Grease and line a 33x23cm/13x9in Swiss roll tin with baking parchment. Whisk the egg whites in a bowl until stiff peaks form when the whisk is removed from the bowl. Add the caster sugar, one teaspoon at a time, whisking between each addition to make a glossy meringue. Cover with cling film and set aside.
Tip the ground almonds and icing sugar into the bowl of a free-standing food mixer and add the whole eggs. Whisk together for 3-5 minutes, or until doubled in volume. Fold in the flour, and then gently fold in the meringue in three separate batches. Pour the melted butter down the side of the bowl and fold in, until incorporated. Tip the mixture into the prepared tin and level the surface with a palette knife. Bake for 5-7 minutes until pale, golden-brown and springy to touch.
Place a sheet of baking parchment over a cooling rack and turn the cake out onto it. Peel off the baking parchment from the base of the cake and leave to cool completely.
For the chocolate ganache, pour the cream into a small pan and heat until just bubbling. Remove from the heat and add the chocolate. Stir until all the chocolate has melted and mixture is smooth and glossy. Pour into a bowl and leave to set.
For the crème au beurre, place the egg yolks in the bowl of a food mixer with the whisk attached. In a pan over a gentle heat, dissolve the sugar in three tablespoons of water and then boil it steadily until the syrup reaches 110C/225F on a sugar thermometer. With the food mixer running, slowly pour the syrup over the egg yolks. Continue whisking until the mixture is thick and mousse-like. Add the butter, a little at a time, whisking continuously and finally add the vanilla paste.
To assemble, slice the cooled sponge horizontally so you have two 33x23cm/13x9in sponges. Cut each of the sponges in half vertically, so that you have four sponges.
Place one of the sponges on a cake board and brush with one-quarter of the kirsch syrup. Spread over one-third of the crème au beurre. Top with another sponge and brush with one-quarter of the kirsch syrup. Spread over the chocolate ganache.
Top with a third sponge and brush with another one-quarter of the kirsch syrup. Spread over another third of the crème au beurre and top with the final sponge. Brush with the remaining kirsch syrup and spread with the remaining crème au beurre.
Arrange the raspberries over the top in rows and brush over the melted jam to glaze. Refrigerate the opera cake for two hours to set the layers and make it easier to trim. Trim the edges using a sharp knife to reveal the layers.
