Opera Cake

Before you start

An opera cake is a flat, rectangular French gâteau of very thin layers. Almond sponge called joconde is soaked with strong coffee syrup and stacked with coffee buttercream and dark chocolate ganache, then covered with a thin, shiny chocolate glaze. Each slice shows clean stripes, and each bite tastes of coffee, chocolate and almond in balance.

The work is easier when you think of it as five small recipes and a careful stack. Bake the joconde as one large sheet and cut it into three equal rectangles. Keep each filling layer thin and level, and chill between layers so they stay distinct. Trimming the edges with a hot knife at the end reveals the stripes that give the cake its look.

Ingredients

• Joconde sponge150 g (1½ cups) finely ground almond flour
• 150 g (1¼ cups) powdered sugar
• 4 large eggs — room temperature
• 40 g (⅓ cup) all-purpose flour
• 4 large (about 120 g) egg whites — room temperature
• 20 g (1½ tbsp) granulated sugar
• 30 g (2 tbsp) unsalted butter — melted and cooled
• Coffee syrup120 ml (½ cup) water
• 100 g (½ cup) granulated sugar
• 2 tbsp instant espresso powder
• Coffee buttercream3 large (about 90 g) egg whites
• 150 g (¾ cup) granulated sugar
• 225 g (1 cup) unsalted butter — soft, cut into cubes
• 1 tbsp instant espresso powder — dissolved in 1 tbsp hot water
• Ganache170 g (6 oz) dark chocolate, 60 to 70% cacao — chopped
• 120 ml (½ cup) heavy cream
• 15 g (1 tbsp) unsalted butter
• Glaze120 g (4 oz) dark chocolate — chopped
• 2 tbsp (30 ml) neutral oil

Method

1. Heat the oven to 220°C (425°F). Line the half-sheet pan with parchment and grease it lightly. Beat the almond flour, powdered sugar and whole eggs on medium-high speed for 5 minutes, until pale and thick. Fold in the all-purpose flour.
2. Whip the egg whites to soft peaks, add the 20 g sugar and whip to firm, glossy peaks. Fold one-third into the almond mixture, then fold in the rest. Fold in the cooled melted butter. Spread evenly over the pan and bake for 6 to 8 minutes, until light golden and springy. Cool, then flip, peel off the parchment and cut into three 15 x 30 cm (6 x 12 in) rectangles.
3. For the syrup, bring the water and sugar to a boil, stir in the espresso powder and cool.
4. For the buttercream, whisk the 3 whites and 150 g sugar in the mixer bowl over simmering water until the mixture reaches 71°C (160°F). Whip on high until the bowl feels cool, about 8 minutes. Add the butter a cube at a time on medium speed, then beat in the dissolved espresso until smooth and silky.
5. For the ganache, pour the simmering cream over the chocolate, wait 2 minutes and stir until glossy. Stir in the butter and let it cool until spreadable but not firm.
6. Place one joconde rectangle on a parchment-lined board and brush it generously with syrup. Spread half the buttercream in an even layer about 5 mm (¼ in) thick. Top with a second rectangle, brush with syrup and spread all the ganache. Chill for 20 minutes.
7. Add the third rectangle, brush with the remaining syrup and spread the rest of the buttercream perfectly level. Chill for 1 hour, until firm.
8. Melt the glaze chocolate with the oil, cool it to about 32°C (90°F) and pour it over the top, spreading quickly in one or two passes. Chill for at least 1 hour. Trim each edge with a knife dipped in hot water and wiped dry, then cut into 12 slices.

Tips for success

• Spread the joconde batter to an even thickness; thin areas overbake into crisp spots that crack when cut.
• Use a ruler when cutting the sponge. Equal rectangles make straight, level layers.
• If the buttercream looks curdled, keep beating; it comes together as the butter and meringue reach the same temperature.

Substitutions

Almond flour

Hazelnut flour makes a nuttier, darker sponge that pairs well with the coffee and chocolate.

Swiss meringue buttercream

A coffee American buttercream is quicker but much sweeter and less silky; use less of it in each layer.

Instant espresso powder

Use 120 ml strong brewed espresso in place of the water in the syrup and skip the powder there. Keep the powder in the buttercream for concentrated flavor.

Variations

Matcha opera cake

Replace the coffee syrup with a matcha syrup and use white chocolate ganache in the middle layer.

Troubleshooting

Layers look uneven when sliced

The fillings were spread at different thicknesses or the cake was not chilled between layers. Use an offset spatula and chill until each layer is firm.

Glaze looks dull or streaky

It was too cool or spread too many times. Pour it at about 32°C (90°F) and smooth it in no more than two passes.

Joconde is dry and cracks

It overbaked. Take it out as soon as it springs back, and cover it with a towel while it cools.

Storage

Keep the cake covered in the refrigerator for up to 4 days. Let slices stand at room temperature for 15 to 20 minutes before serving so the buttercream softens. The whole cake freezes well for up to 1 month before glazing.

Safety notes

Do not taste the raw sponge batter; it contains raw egg and flour. The buttercream whites are heated to 71°C (160°F). Contains tree nuts (almonds), wheat, egg and milk. Coffee adds a small amount of caffeine.