So Gateau Opera is one of those desserts that I have read a lot about, but never really bothered to try making on my own. I have made the sponge, jaconde, before, and obviously, I have made ganache before. But the other filling, which is a coffee infused French buttercream, not so much. For those of you who do not know what French buttercream is, it is a sabayon, or thick, rich, egg yolk custard, that you emulsify with butter to form almost like a mousse for a cake. My stance on traditional American buttercream is that it is heavy, cloying, and honestly, kind of gross. When you give me butter and sugar, my immediate thought process is to use these ingredients to make a pastry, dough, or batter, not to whip them together and pipe them or use them for filling. That’s giving somebody diabetes in the wrong way. However, meringue-based and French buttercreams are my two exceptions. While they are called buttercream, they are light, airy, and quite honestly, delicious, contrary to their gross American counterpart.
While I have never really made a French buttercream before making this, I actually, not to humblebrag, did not even need to research how to make one. I have made sabayon before, and I have finished them with butter, so I know that for that buttercream, all I really had to do was just add a ton more butter. I figured a ratio of 3 egg yolks to 2 sticks of butter would be good, since with my Italian meringue buttercream, I would do that same ratio, but with whites instead. And luckily for me, it worked exactly like how I was hoping it would! Overall, for me anyways, the biggest challenges with this cake is making sure that the layers are even, that you assemble it carefully, and that you slice away just enough cake so that you still have a beautiful gateau. The top layer in particular, you need to have a lot of thought going into it to make sure that the ganache is shiny, smooth, and not gross looking. I garnished mine using tempered chocolate that I spread onto acetate sheets, just for a shiny, dramatic finish!
3 whole eggs
3 egg whites
1 1/4 cups almond flour
1/3 cups all-purpose flour
1/2 cup granulated sugar
2 teaspoons canola oil
a pinch of salt
1 teaspoon vanilla extract
Whip whole eggs with half the sugar. Whip egg whites with the rest of the sugar, salt, and vanilla. Sift flours together. Once the whole eggs reach ribbon stage and the egg whites reach stiff peaks, fold everything together with the oil. Spread evenly on three silicone baking sheets and bake each sheet at 350 degrees F for 18 minutes. Once baked, cool completely before removing from the baking sheets.
1/2 cup granulated sugar
1/2 cup water
1/4 cup instant espresso
a pinch of salt
Combine ingredients and bring to a simmer. Reserve 1/4th of the syrup for the soaking liquid. Use the rest for the buttercream.
3 egg yolks
2 sticks unsalted butter; cubed and softened
Whip egg yolks with the warm simple syrup until the egg yolks reach ribbon stage. At that point, slowly whisk in the butter, little by little at first to prevent the egg yolks from breaking, before adding in more gradually. As the mixture emulsifies, you can increase your whisking speed. Continue whisking until the buttercream is fully mixed together. Should it break, simply dip the bottom of the mixing bowl into some warm water before whisking again, using the heat to help with the emulsification.
1/2 cup heavy cream
2/3 cups dark chocolate chips
4 tablespoons unsalted butter
a pinch of salt
Heat heavy cream, salt, and chocolate together until melted. Take off heat and whisk in the unsalted butter until melted. Split ganache into three parts. Refrigerate two of the three parts to bring the temperature of the chocolate down to just above room temperature. Keep one layer at room temperature, if not slightly warmer, and make sure that it is still liquid.
Brush coffee syrup onto all three layers. Spread the French buttercream evenly on two layers. Refrigerate these layers for at least 10 minutes. Take out and then pour on the two parts of refrigerated ganache. Spread these as well, and then set for another 10 minutes in the refrigerator or freezer. Pour the remaining third part of warmed ganache onto the last of the three cooled cake layers, spreading it with either an offset or simply by tilting the cake, or tapping the cake in a sheet tray against a countertop, just to prevent the top layer from having any ripples. If you do have ripples, you can use a blowtorch or hairdryer to gently reheat the ganache so that it can naturally spread out. To finish your assembly, place the two buttercream-ganache layers on top of one another, then place the evenly spread ganache layer on top of that one. Refrigerate for at least 30 minutes, then with a warm knife, cut off the edges until you have an even square or rectangular cake, making sure to wipe the knife with a warm, wet towel between cuts. Refrigerate again until you are ready to garnish and serve.
In a double boiler, melt dark chocolate with sea salt. Once the dark chocolate reaches 95 degrees F, take off heat and stir in about 1/4th of the amount of dark chocolate that you had already melted. Once that is mixed in, spread onto acetate sheets, then drag a fork along the surface of the chocolate to form curls. Roll up the acetate and freeze for at least 5 minutes before removing. Use either plating tweezers or a fork to transfer the chocolate onto the cake, just to not tarnish the shiny finish or for your fingers to melt the chocolate as your plate it on your finished cake!