Opera Cake - Gâteau Opéra

Preparation time: 2 to 3 hours

10 to 12 servings

Genoise au chocolat
165g eggs, at room temperature
90g caster sugar
70g all purpose flour
20g dutch cocoa powder

Biscuit Joconde
165g icing sugar
165g almond meal/flour
220g eggs, at room temperature
40g all purpose flour
105g egg withes, at room temperature
30g caster sugar
50g unsalted butter, melted at room temperature

Chocolate Ganache
700g heavy cream
300g high quality semi sweet chocolate, chopped
400g high quality dark chocolate, chopped

Crème Mousseline au Café
165g eggs, at room temperature
108g egg yolks, at room temperature
330g caster sugar
100g water, at room temperature
400g unsalted butter, at room temperature

Coffee extract

Coffee Syrup
Prepare a syrup with water, sugar and coffee.

Glaçage Miroir au Chocolat
300g chocolate ganache
60g glucose syrup
A few drops of red gel food colouring 

1.  Genoise au chocolat: Preheat the oven to 190°C (375°F). Line a sheet pan (30cm x 40cm) with a Silpat or spray the pan lightly with nonstick spray, line with parchment paper, and spray the parchment. Sift the flour, baking powder, and cocoa powder in a medium bowl and set aside.
2. Place the eggs and sugar into a large heatproof bowl over pot of simmering water. Beat them constantly until it reach a 45°C temperature, or until the sugar cristal had been dissolved. Place the mixture in the bowl of a stand mixer fitted with the whisk attachment and mix on medium-high speed for about 10 minutes.
3. Transfer the mixture to a large bowl and fold the dry ingredients in 3 additions into the batter. Pour the batter into the prepared pan and, using an offset spatula, spread it in an even layer, making sure that it reaches into the corners. Bake for about 10 to 12 minutes, until a skewer inserted in the center comes out clean. Set on a cooling rack and cool completely.
4. Biscuit Joconde: Preheat the oven to 190°C (375°F). Line a sheet pan (30cm x 40cm) with a Silpat or spray the pan lightly with nonstick spray, line with parchment paper, and spray the parchment. Sift the flour in a bowl and set aside.
5. Place the eggs, almond flour, and icing sugar in the bowl of a stand mixer fitted with the whisk attachment and mix on medium-speed until light and fluffy. Transfer the mixture to a large bowl and set aside.
6. Whip the egg whites in a clean bowl with the clean whisk attachment on medium speed for about 45 seconds, until foamy. Add slowly the sugar then increase to medium-high speed and whip until the whites are glossy with soft peaks. Do not over beat!
7. Fold the flour and egg whites gently into the egg mixture in 3 additions, starting with the egg whites and finishing with them. Whisk about one cup of the batter into the melted butter and then fold gently with the remaining batter. Pour the mixture into the prepared pan and, using an offset spatula, spread it in an even layer, making sure that it reaches into the corners. Bake for about 10 to 12 minutes, until a skewer inserted in the center comes out clean. Do not over bake. Set on a cooling rack and cool completely.
8. Ganache: In a small saucepan, bring the cream just to a boil. Pour over the chocolate and let it sit for about 2 minutes. Whisk the mixture until completely smooth. Reserve 300g of the ganache for the Glaçage Miroir au Chocolat and cover with plastic wrap. Place the remaining ganache in the refrigerator and stir from time to time (10 minutes intervals).
9. Crème Mousseline au Café: Place the eggs, egg yolks, and 1 tablespoon of sugar int he bolw of a stand mixer fitted with the whisk attachment and mix on medium speed for about 10 minutes. Make a syrup with the water and the remaining sugar until it reaches 118°C. When the syrup is ready, remove the pan from the heat. Turn the mixer to medium-high speed, and slowly ad the syrup, pouring it between the side of the blow and the whisk. Whip the mixture until it is at room temperature.
10. Reduce the speed to medium and add the butter, a few pieces at a time. If at any point the mixture looks broken, increase the speed and beat to re-emulsify it, then reduce the speed and continue adding the butter. Add the coffee extract and mix until it is well incorporated. Transfer the mixture to a covered container and place in the refrigerator until you need it.

To assembly the cake:

1. Divide the Biscuit au Chocolat and the Genoise in two parts each. Put 1 biscuit au chocolat square on a plate, then brush generously with one quart of coffee syrup. Spread one third of ganache evenly over top with a cleaned offset spatula, spreading to the edges. Transfer to the refrigerator until it firms.
2. When it sets, spread one third of the coffee mousseline evenly over top with a cleaned offset spatula, spreading to the edges. Put 1 genoise square over the mousseline then brush with another part of the syrup. Spread another layer of the coffee mousseline and transfer to the refrigerator until it firms.
3. When it sets, spread another portion of the ganache. Put the second square of the biscuit and brush with the syrup, cover with the ganache again (this time make a very thin ganache layer - half of the amount that you did on the other layers) and transfer again to the refrigerator. After that cover with the remaining mousseline and return to the fridge to make the miroir.
4. Miroir au chocolat: Place the reserved ganache (300g) with the other ingredients in a saucepan and bring to medium heat until everything is well incorporated. Remove pan from the heat and let cool slightly. It should be like a sauce, but not too hot because it could melt the mousseline.
5. Cover the cake with the chocolate glaze and chill cake until glaze is set, about 30 minutes, then trim edges slightly with a sharp serrated knife.

Opéra cake can be made 2 days ahead. Cover sides with strips of plastic wrap and top of cake loosely with plastic wrap (once glaze is set) and chill cake. Remove plastic wrap from top immediately after removing cake from refrigerator and serve it.