The Daring Bakers Challenge: Opera Cakes

May was my first month as part of the Daring Bakers group, and what an introduction! Our challenge this month was to make an opera cake. Opera cakes usually involve thin layers of sponge cake alternating with chocolate or coffee buttercream and mousse, but the challenge specified "no dark colors or flavors," so I decided to make a mint-raspberry version, with a light mint buttercream and a delicious raspberry mousse.
I also wanted to make mini round cakes. I'm obsessed with miniature food (everything tastes better smaller!) and love the neatness of individual round cakes. So after the joconde was made, I cut it out into 3" circles. Since each cake would have three layers of sponge, I ended up with 24 circles, or enough for 8 cakes. I didn't have pastry rings to assemble the cakes in, so I used the next best thing. And by "next best thing," I mean the cheapest, most convenient thing: heavy stationary. Posterboard would have been better, but we work with what we have, right? I cut heavy cardstock into strips, wrapped it around my bottom layer (already on small cardboard circles) and taped it nice and tight. As you can see from the picture, I went a little crazy and made them unnecessarily tall.
After I did the first one, I figured out the proper height for the rings and cut the rest of them down so that filling was much more manageable. The assembly wasn't bad at all, actually. I put the mousse and buttercream in pastry bags, so that I could easily pipe even layers into the rings. After assembly they chilled in the refrigerator for a few hours, so that when I applied the warm glaze they wouldn't melt. I finished the cakes with fresh mint leaves, fresh raspberries, a rosette of whipped cream, and a white chocolate treble clef. I was worried about unmolding them but they held their shape beautifully.
The best part is, they taste as good as they look! The buttercream recipe called for a lot of butter, and unfortunately I only had salted butter, so when I tasted it by itself it seemed a bit salty. However, the white chocolate glaze was very sweet, so the slightly tart mousse and slightly salty buttercream balanced it out nicely. As for the glaze, I would definitely increase the cream next time, since it hardened a bit too much for my liking. But the cake was deliciously moist and tender, the mousse was divine, and the buttercream was oh so rich with just the right amount of refreshing mint flavor.
Here's the (world's longest) recipe:

Joconde

• 6 large egg whites, at room temperature
• 2 tbsp. (30 grams) granulated sugar
• 2 cups (225 grams) ground blanched almonds
• 2 cups icing sugar, sifted
• 6 large eggs
• ½ cup (70 grams) all-purpose flour
• 3 tbsp. (1½ ounces; 45 grams) unsalted butter, melted and cooled

Divide the oven into thirds by positioning a rack in the upper third of the oven and the lower third of the oven. Preheat the oven to 425^◦F. (220^◦C). Line two 12½ x 15½- inch (31 x 39-cm) jelly-roll pans with parchment paper and brush with melted butter.

In the bowl of a stand mixer fitted with the whisk attachment (or using a handheld mixer), beat the egg whites until they form soft peaks. Add the granulated sugar and beat until the peaks are stiff and glossy. If you do not have another mixer bowl, gently scrape the meringue into another bowl and set aside.

If you only have one bowl, wash it after removing the egg whites or if you have a second bowl, use that one. Attach the paddle attachment to the stand mixer (or using a handheld mixer again) and beat the almonds, icing sugar and eggs on medium speed until light and voluminous, about 3 minutes.

Add the flour and beat on low speed until the flour is just combined. Using a rubber spatula, gently fold the meringue into the almond mixture and then fold in the melted butter. Divide the batter between the pans and spread it evenly to cover the entire surface of each pan.

Bake the cake layers until they are lightly browned and just springy to the touch. This could take anywhere from 5 to 9 minutes depending on your oven. Place one jelly-roll pan in the middle of the oven and the second jelly-roll pan in the bottom third of the oven.

Put the pans on a heatproof counter and run a sharp knife along the edges of the cake to loosen it from the pan. Cover each with a sheet of parchment or wax paper, turn the pans over, and unmold. Carefully peel away the parchment, then turn the parchment over and use it to cover the cakes. Let the cakes cool to room temperature.

Simple Syrup

• ½ cup (125 grams) water
• ⅓ cup (65 grams) granulated sugar

Stir all the syrup ingredients together in the saucepan and bring to a boil. Remove from the heat and let cool to room temperature.

Buttercream

• 1 cup granulated sugar
• 1/4 cup water
• 1 large egg
• 1 large egg yolk
• 1 tsp mint extract
• 7 ounces unsalted butter

Combine the water and sugar in a small saucepan and warm over medium heat just until the sugar dissolves. Continue to cook, without stirring, until the syrup reaches 240 (softball stage) on a candy or instant-read thermometer. Once it reaches that temperature, remove the syrup from the heat.

While the syrup is heating, begin whisking the egg and egg yolk at high speed in the bowl of your mixer using the whisk attachment. Whisk them until they are pale and foamy.

When the sugar syrup reaches the correct temperature and you remove it from the heat, reduce the mixer speed to low speed and begin slowly (very slowly) pouring the syrup down the side of the bowl being very careful not to splatter the syrup into the path of the whisk attachment.

Raise the speed to medium-high and continue beating until the eggs are thick and satiny and the mixture is cool to the touch (about 5 minutes or so).

While the egg mixture is beating, place the softened butter in a bowl and mash it with a spatula until you have a soft creamy mass.

With the mixer on medium speed, begin adding in two-tablespoon chunks. When all the butter has been incorporated, raise the mixer speed to high and beat until the buttercream is thick and shiny.

Add the mint extract and green food coloring (optional) until well-incorporated.

Mousse

• 2 teaspoons unflavored powdered gelatin
• 2 tablespoons cold water
• 5 cups fresh raspberries or two 10-ounce bags frozen raspberries, thawed
• 3/4 cup sugar
• 2 large egg whites, at room temperature
• 1 cup heavy cream

In a small bowl, sprinkle the gelatin over the water and let stand until softened, about 5 minutes. In a blender, puree 4 cups of the raspberries with 1/2 cup of the sugar. Strain the puree into a bowl through a fine sieve.

In a microwave oven, melt the gelatin for 10 seconds on low power. Whisk the gelatin into the raspberry puree.

In a large bowl set over a pot of simmering water, whisk the egg whites with the remaining 1/4 of cup sugar until warm to the touch. Remove the bowl from the heat. Using a hand-held electric mixer, beat the whites at medium-high speed until stiff and glossy. Fold the egg whites into the puree.

In the same bowl, beat the cream until firm. Fold the whipped cream into the raspberry mixture. In a small bowl, coarsely mash the remaining 1 cup of raspberries. Fold them into the mousse.

Glaze

• 14 ounces white chocolate, coarsely chopped
• ½ cup heavy cream (35% cream)

Melt the white chocolate with the heavy cream. Whisk the mixture gently until smooth. Let cool for 10 minutes and then pour over the chilled cake. Using a long metal cake spatula, smooth out into an even layer. Place the cake into the refrigerator for 30 minutes to set.