Step 1: Preheat your oven to 450°F (230°C). Line two half-sheet pans (13x18 inches) with parchment paper. Lightly grease the parchment to ensure easy removal.
Step 2: In a large bowl, sift together 6 large egg whites and 1/4 cup (50g) granulated sugar. Whip with an electric mixer on medium-high speed until stiff, glossy peaks form. Set aside.
Step 3: In another large bowl, combine 6 large whole eggs, 1 1/2 cups (150g) almond flour, 1/2 cup (60g) powdered sugar, and 1/4 cup (30g) all-purpose flour. Beat on medium speed until light and airy, about 5 minutes.
Step 4: Melt 2 tablespoons (28g) unsalted butter. Gently fold the melted butter into the almond mixture.
Step 5: Carefully fold about one-third of the whipped egg whites into the almond mixture to lighten it, then gently fold in the remaining whites until just combined, being careful not to deflate the mixture.
Step 6: Divide the batter evenly between the two prepared sheet pans. Spread thinly and evenly with an offset spatula. Bake for 5-7 minutes, or until lightly golden and set. The cakes will be thin. Immediately invert onto clean sheets of parchment paper, peel off the baking parchment, and let cool. Trim the edges to create four equal rectangles, approximately 9x4 inches each.
Step 7: In a small saucepan, combine 1/2 cup (120ml) water, 1/2 cup (100g) granulated sugar, and 2 tablespoons (14g) instant espresso powder. Bring to a simmer over medium heat, stirring until the sugar and espresso are dissolved. Remove from heat and let cool completely.
Step 8: In a heatproof bowl set over a saucepan of simmering water (bain-marie), combine 3 large egg yolks, 1/2 cup (100g) granulated sugar, and 1/4 cup (60ml) water. Whisk constantly until the mixture thickens slightly and reaches 160°F (71°C) on an instant-read thermometer.
Step 9: Remove from heat and transfer to a stand mixer bowl. Beat on high speed with the whisk attachment until the mixture is very pale, thick, and has cooled completely, about 7-10 minutes.
Step 10: Switch to the paddle attachment. Gradually add 1 1/2 cups (340g) softened unsalted butter, one tablespoon at a time, beating well after each addition until fully incorporated and the buttercream is light and fluffy. Add 2 teaspoons (7g) instant espresso powder dissolved in 1 teaspoon hot water (or 1 tbsp strong cooled coffee). Continue beating until smooth and creamy. If it separates, keep beating; it will come back together.
Step 11: Finely chop 8 ounces (225g) good quality dark chocolate. Place in a heatproof bowl.
Step 12: Heat 1 cup (240ml) heavy cream in a small saucepan over medium heat until it just begins to simmer. Pour the hot cream over the chopped chocolate. Let sit for 5 minutes, then whisk gently from the center outwards until smooth and glossy. Stir in 2 tablespoons (28g) unsalted butter until melted and combined. Let cool to room temperature, stirring occasionally, until it reaches a spreadable consistency.
Step 13: Place one trimmed joconde sponge rectangle on your serving platter or a sturdy cardboard base. Brush generously with coffee syrup.
Step 14: Spread about half of the coffee buttercream evenly over the sponge layer. Place another joconde sponge layer on top, pressing gently. Brush generously with coffee syrup.
Step 15: Spread about half of the chocolate ganache evenly over the second sponge layer. (If the ganache is too firm, warm slightly; if too soft, chill briefly.) Place a third joconde sponge layer on top, pressing gently. Brush generously with coffee syrup.
Step 16: Spread the remaining coffee buttercream evenly over the third sponge layer. Place the final joconde sponge layer on top, pressing gently. Brush generously with coffee syrup.
Step 17: Spread the remaining chocolate ganache evenly over the top layer of sponge. Use an offset spatula to create a smooth, even finish.
Step 18: Chill the cake in the refrigerator for at least 2-4 hours, or ideally overnight, to allow the layers to set and the flavors to meld. This step is crucial for clean slicing.
Step 19: Once chilled, carefully trim the edges of the cake with a hot, sharp knife (dip the knife in hot water and wipe clean between cuts) to reveal the beautiful layers. You can pipe 'Opera' on top with melted chocolate or simply leave it as is. Enjoy!