Make the cake: Heat your oven to 350°F (175°C). Line the bottom of two 9×13-inch pans with parchment paper and coat the parchment paper (yes, this extra buffer helps a lot) and the exposed sides of the pan with nonstick spray.
In a mixer bowl fitted with a whisk attachment, whip the 4 egg whites on medium speed until they’re halfway thickened — they should look white, foamy, and show some streaks as the whisk moves through them in the bowl. Running the mixer the whole time, sprinkle in salt, then 1 1/2 tablespoons (20 grams) of the sugar. Increase the speed and beat the egg whites until glossy, stiff peaks form, about 2 minutes more. Scoop the whipped egg whites into a second bowl for now, and return the empty mixing bowl to the electric mixer. [No need to wash the bowl or whisk here.]
Add 4 whole eggs and remaining 3/4 cup (150 grams) sugar to the empty bowl and whip together on high speed until light in color and texture and and thickened ripples form in the batter as the whisk moves through the bowl, about 8 to 10 minutes. If you’re unsure, it doesn’t hurt to beat the mixture a minute longer. Remove the bowl from the mixer and fold in the flour with a flexible spatula, followed by the almond flour, half at a time. Finally, fold in the whipped egg whites, being careful not to deflate the batter.
Divide the batter between the two prepared pans and spread evenly with an offset spatula. Bake the cakes until they’re set, about 9 to 13 minutes. Transfer cakes to cooling racks and let them cool completely in the pan. However, I like to run a thin knife carefully around the cake’s sides before it cools as it has a tendency to stick, even when greased well.
Make the buttercream: In a medium saucepan, combine 1/2 cup water and 3/4 cup sugar, then turn heat to high and boil, cooking (without stirring) until mixture reaches 238°F (114°C) (aka the soft ball candy stage) and remove from the heat.
In a mixer bowl fitted with a whisk attachment, beat egg yolks for 2 minutes on a medium-high speed to loosen them. With the mixer running, add the hot sugar syrup to the yolks in a slow, thin stream (if you can, aim for the middle as the sugar syrup that lands on the sides of the bowl just hardens there), beating until combined. Add the espresso and beat until combined. Let the mixture cool to close to room temperature. You can hurry this along, as I like to, by placing some ice packs around the mixing bowl and running the machine on low (so it cools evenly and not just at the edges).
Once cool, with the mixer running, add pieces of softened butter, one at a time. The mixture may begin to look curdled – this is normal, it will be fine. When all of the butter has been added, add the salt and beat on high speed until the mixture comes together in a thick, creamy, silky buttercream that will put all other frostings to shame.
Congratulations, you just finished the two peskiest elements of the cake! The next two parts are a cinch.
Make the chocolate layer: Place chopped chocolate in a medium bowl. Bring cream to simmer. Pour cream over, wait one minute, then whisk until chocolate has melted. You can cool and thicken this quickly by setting this bowl over a larger bowl partially filled with ice water and whisking until the chocolate mixture is almost thick enough to spread. Remove the bowl from the bowl of ice water. You can stop a little short of fully thickened because it will continue to firm up as it rests.
Make the syrup: Combine sugar and hot espresso, stirring until sugar is dissolved. Add brandy and set aside.
Assemble the cake: Run a knife (again) around the cooled cakes to make sure they’re not stuck and use the parchment underneath to sort of yank the first cake, parchment and all, onto a cutting board. Use a ruler (you will thank you later because evenly cut layers need less trimming) to evenly divide the cake layer into thirds, cutting through the parchment and all. Repeat with the second cake layer, creating 6 9-inch by (approximately) 4 1/3-inch rectangles of cake.
Pick up the first cake layer and flip it briefly over onto the cutting board to peel the parchment carefully off the back. Place the first cake layer on your serving plate. Don’t worry, the cake is sticky but bendy; it shouldn’t break but even if it did, nobody would be the wiser once you patched it back together on the plate.
Brush generously with 1/6 of the soaking syrup, just eyeballing the amount. Spread with 1/4 of the chocolate mixture. Place the second cake layer with the parchment peeled off as before on the chocolate layer. Brush generously with another 1/6 of the soaking syrup. Spread with 1/3 of the buttercream mixture. You’ve just completed the first third of the total cake. You want to repeat this process two more times — with 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 a freezer for 10, just until the buttercream on top is firm to the touch.
Once the buttercream on top is firm to the touch, spread the remaining chocolate on top. If you’d like to decorate the top, hold back 2 tablespoons of the chocolate to pipe what you wish. It is traditional to write “opera” on top. I, uh, wrote “smitten.” Happy birthday works too!
To finish the cake: Let the cake rest in the fridge for a couple hours, or overnight, so that it’s firm and easy to trim. Before you’re ready to serve it, use a sharp serrated knife to trim the edges of the cake so that nice, clean stripes are visible. Serve right away; you’ve waited long enough!
Do ahead: The cake keeps in the fridge for 5 days, although it would be unprecedented in my apartment. I suspect it would also freeze nicely, once firm enough to wrap tightly in plastic.
Additional Notes:
“Deb, why don’t we just flip the cakes out of the pan onto a cooling rack and then remove the parchment paper off the back of the cakes, as we would with a regular layer cake?” Because the cakes are SO sticky. I did it this way a few times and it created an extra unnecessary headache to then try to take the cake off the rack without losing half of it.
Don’t want to use alcohol in the soaking syrup? No worries, just use extra espresso.
For years, I’d order the Opera Cake from Balthazar bakery for my birthday and one thing I like that they do is they make the cake with hazelnut flour/meal instead of almonds. It really goes nicely here if you want to make the swap here.
