Make the cake: Heat oven to 350°F (175°C). Line the bottoms of two 9-by-13-inch pans with parchment paper, then coat the parchment paper (yes, this extra cushioning helps a lot) and the exposed sides of the molds with nonstick spray.
In a mixer bowl fitted with a whisk attachment, beat the 4 egg whites on medium speed until medium-thick. As the whisk moves around the bowl, it should become white and foamy, with visible streaks. With the mixer running all the way, sprinkle with salt, then 1 1/2 tablespoons (20 grams) of sugar. Increase speed and beat egg whites until shiny, stiff peaks form, about 2 minutes more. Scoop the whipped egg whites into a second bowl for now, then return the empty mixing bowl to the electric mixer. (No need to wash or whisk the bowl.)
Add 4 whole eggs and remaining 3/4 cup (150 grams) sugar to the empty bowl and beat on high speed until the color and texture lightens and thick ripples form in the dough as the whisk moves through the bowl, about 8 to 10 minutes. If you are unsure, it is okay to mix the mixture for 1 minute longer. Remove the bowl from the mixer and fold in the flour with a flexible spatula, followed by the almond flour, half and half. Finally, add the whipped egg whites and mix, being careful not to deflate the dough.
Divide the batter between the two prepared pans and spread evenly with an offset spatula. Bake until cake is set, about 9 to 13 minutes. Transfer the cake to a cooling rack and let cool completely in the pan. However, even with a good oiling, they tend to stick, so I like to carefully run a thin knife around the sides of the cake before it cools.
Make the buttercream: Combine 1/2 cup water and 3/4 cup sugar in a medium saucepan, increase heat to high, bring to a boil, and cook (without stirring) until mixture reaches 238°F (114°C) (aka softball candy stage), then remove from heat.
In a mixer bowl fitted with a whisk attachment, beat the egg yolks on medium-high speed for 2 minutes to loosen them. With the mixer running, slowly add the hot sugar syrup to the egg yolks in a thin stream (preferably in the middle, as the sugar syrup will solidify on the sides of the bowl). Mix until combined. Add espresso and mix until combined. Cool the mixture to near room temperature. You can hurry this along, as I like to do, by putting some ice packs around the mixing bowl and running the machine on low (so it’s evenly chilled, not just the edges).
Once cool, add the softened butter, one piece at a time, with the mixer running. The mixture may appear curdled; this is normal and okay. Once all the butter is added, add the salt and mix on high speed until you get a rich, creamy, silky buttercream that rivals any other frosting.
congratulations. The two trickiest elements of the cake are now complete. The next two parts are easy.
Make the chocolate layer: Place the chopped chocolate in a medium bowl. Bring the cream to a boil. Pour in the cream, wait 1 minute, and whisk until the chocolate is melted. You can quickly cool and thicken this by placing this bowl over a large bowl partially filled with ice water and whisking until the chocolate mixture is almost fully spread. Remove the bowl from the bowl of ice water. If you leave it for a while, it will continue to solidify, so you can stop before it gets completely thick.
Make the syrup: Combine sugar and hot espresso and stir until sugar dissolves. Add brandy and set aside.
Assemble the cake: Run a knife around the cooled cake (again) to make sure it doesn’t stick, then use the parchment paper underneath to pull the first cake, parchment paper and all, onto a cutting board. Using a ruler (you’ll thank me later, because cutting the layers evenly means you’ll need less trimming), divide the cake layers into thirds evenly and cut out the entire parchment. Repeat with the second cake layer, creating six 9-inch x (approximately) 4 1/3-inch rectangular cakes.
Pick up the first layer of cake, gently invert it onto a cutting board, and carefully remove the parchment paper from the underside. Place the first cake layer on a plate. Don’t worry, the cake will be sticky, but it will bend. It shouldn’t break, but even if it does, no one will be the wiser if you patch it back up on the plate.
Brush generously with 1/6 of the soaked syrup. Spread with 1/4 of the chocolate mixture. As before, remove the parchment paper and place the second cake layer on top of the chocolate layer. Brush generously with another 1/6 of the soaked syrup. Brush with 1/3 of the buttercream mixture. The first third of the cake is now complete. Repeat this process two more times. Add 4 more cake layers, 2/3 of the remaining chocolate and all of the remaining buttercream. Transfer this to the fridge for 30 minutes or the freezer for 10 minutes until the buttercream on top is firm to the touch.
Once the buttercream on top has set, spread the remaining chocolate on top. If you want to decorate the top, save about 2 tablespoons of the chocolate and add your favorite decorations. It is traditional to write “opera” at the top. I wrote “obsessed.” Happy Birthday also works!
To finish the cake: Place the cake in the refrigerator for a few hours or overnight to firm up and make it easier to cut. Before serving, use a sharp serrated knife to trim the edges of the cake to reveal nice, clean stripes. Serve immediately. I’ve waited long enough!
Do it first: The cake can be stored in the refrigerator for 5 days, which is unheard of in my apartment. I think once it’s hard enough to wrap tightly in plastic, it also freezes well.
Additional notes:
“Deb, why don’t you flip the cake out of the pan onto a cooling rack and then peel the parchment paper off the back of the cake like you would with a regular layer cake?” Because the cake is so sticky. I repeated this method several times and it caused an extra headache trying to get the cake off the rack without losing half of it. Don’t want to use alcohol in your steeping syrup? Don’t worry, use extra espresso. For years, I have been ordering Balthazar Bakery’s opera cake for my birthday, and one of my favorite things about it is that they use hazelnut flour/meal instead of almonds to make the cake. If you want to exchange here, it works fine here.
