Step 1: Preheat your oven to 400°F (200°C). Line two large baking sheets (approx. 13×18 inches or similar) with parchment paper. Lightly grease the parchment to ensure it sticks.
Step 2: In a large bowl, whisk together the almond flour, powdered sugar, and all-purpose flour. Set aside.
Step 3: In a separate bowl, whisk 6 large egg yolks until pale. Gradually add the melted and cooled butter, mixing until combined. Fold the egg yolk mixture into the dry ingredients until just combined. This forms a thick paste.
Step 4: In a clean, dry bowl, whip the 6 egg whites with an electric mixer until soft peaks form. Gradually add the 30g granulated sugar and continue whipping until stiff, glossy peaks form (meringue).
Step 5: Gently fold one-third of the meringue into the almond mixture to lighten it, then carefully fold in the remaining meringue until no streaks remain. Be careful not to deflate the mixture.
Step 6: Divide the batter evenly between the two prepared baking sheets. Spread it into thin, even layers using an offset spatula. Bake for 5-8 minutes, or until the sponges are lightly golden and spring back when gently touched. Immediately invert the baked sponges onto a wire rack lined with parchment paper, peel off the baking parchment, and let cool completely. Once cool, trim the edges to create two perfect rectangular sheets.
Step 7: In a small saucepan, combine the strong brewed espresso (or coffee) and granulated sugar. Heat over medium-low heat, stirring until the sugar is completely dissolved. Remove from heat and let cool. If using, stir in the coffee liqueur once cooled.
Step 8: In a heatproof bowl set over a saucepan of simmering water (bain-marie), whisk the 3 egg yolks, 100g granulated sugar, and 50ml water together. Whisk constantly until the mixture thickens, reaches 160°F (71°C), and leaves a ribbon trail when the whisk is lifted (this creates a pâte à bombe base).
Step 9: Remove the bowl from the heat and continue to beat the mixture with an electric mixer until it’s very pale, thick, and completely cooled to room temperature. This can take 10-15 minutes.
Step 10: In a separate bowl, beat the softened butter until light and fluffy. Gradually add the cooled egg yolk mixture to the butter, beating well after each addition until fully incorporated and smooth. Beat in the dissolved espresso powder (or strong espresso) until uniformly flavored.
Step 11: Place the chopped bittersweet chocolate in a medium heatproof bowl. In a small saucepan, heat the heavy cream until it just begins to simmer. Pour the hot cream over the chocolate and let it sit for 2-3 minutes. Whisk gently from the center outwards until the chocolate is fully melted and smooth. Stir in the softened butter until fully incorporated and glossy. Let the ganache cool slightly until it’s spreadable but not too firm.
Step 12: Prepare your cake board or serving platter. Cut one of your Joconde sponges in half lengthwise, creating two thinner rectangular pieces. You should now have three pieces of sponge: one full sheet and two half sheets.
Step 13: Place one full Joconde sponge sheet on your serving platter. Generously brush it with about one-third of the coffee syrup until thoroughly moistened.
Step 14: Spread half of the coffee buttercream evenly over the first sponge layer. Chill in the refrigerator for 15-20 minutes to set.
Step 15: Carefully place one of the half-size Joconde sponges on top of the buttercream. Brush generously with another third of the coffee syrup.
Step 16: Spread the chocolate ganache evenly over this sponge layer. Chill again for 15-20 minutes to set.
Step 17: Place the final half-size Joconde sponge on top of the ganache. Brush with the remaining coffee syrup.
Step 18: Spread the remaining coffee buttercream evenly over this sponge layer. Chill for at least 30 minutes, or until firm.
Step 19: Place the chopped bittersweet chocolate in a medium heatproof bowl. In a small saucepan, heat the heavy cream and corn syrup until it just begins to simmer. Pour over the chocolate and let sit for 2-3 minutes. Whisk gently until smooth. Stir in the softened butter until fully incorporated and glossy. Let the glaze cool slightly until it reaches about 95-100°F (35-38°C) – it should be pourable but not hot.
Step 20: Pour the slightly cooled glaze over the chilled cake, spreading it quickly and evenly with an offset spatula to cover the top. Allow any excess to drip off. Smooth the top if needed, working quickly before it sets.
Step 21: Chill the Opera Cake for at least 4 hours, or preferably overnight, to allow all the layers to set properly and flavors to meld. When ready to serve, use a very sharp knife (dipped in hot water and wiped clean between each slice) to cut the cake into desired portions.