Preheat the oven to 200°C (390°F). Line a rimmed sheet pan with parchment and set aside.
Make the joconde sponge: in a bowl, combine the Almond Flour with the Icing Sugar. Beat in the Eggs until smooth, then sift in the Cake Flour and fold gently to combine.
Whip the meringue: in a separate clean bowl, whip the Egg Whites with a pinch of Salt until foamy, then gradually add the Granulated Sugar and beat to glossy, stiff peaks. Fold one third of the meringue into the almond mixture to loosen it, then gently fold in the remainder until airy and homogeneous.
Melt the Unsalted Butter and cool slightly; fold it into the batter in a thin stream to enrich and loosen the mix. Spread the batter evenly in the prepared pan to form a thin layer and smooth the surface.
Bake the sponge for about 8–12 minutes, until just springy and lightly golden. Remove from oven, cool briefly in the pan, then turn out onto a rack and trim to even rectangles while still warm.
Prepare the coffee syrup: combine brewed Espresso with Water and a little Granulated Sugar in a small saucepan; warm until the sugar dissolves. If you want an intensified coffee note, stir in a pinch of Espresso Powder. Cool completely.
Make the moka buttercream: in a small saucepan combine Granulated Sugar and Water and bring to a simmer until the sugar reaches about 118°C (245°F). Meanwhile, whisk the Egg Yolks until pale. Slowly pour the hot sugar syrup into the yolks while whisking, then beat the mixture until thick and cooled. Gradually incorporate room-temperature Unsalted Butter in pieces until the buttercream is smooth and silky, then fold in some of the cooled Espresso or dissolved Espresso Powder to taste.
Make the chocolate ganache: chop the Dark Chocolate and place in a heatproof bowl. Heat the Heavy Cream until steaming, pour over the chocolate and let sit a minute; stir until smooth. Whisk in a little Unsalted Butter and a splash of Neutral Oil for gloss and spreadability.
Assemble the cake: place one trimmed joconde rectangle on your work surface and brush generously with the coffee syrup. Spread a thin layer of ganache, top with another soaked joconde, then spread an even layer of moka buttercream. Repeat to build 3–4 alternating layers (sponge/ganache/sponge/buttercream), finishing with a thin, smooth layer of ganache on top. Press gently to even the layers and chill until firm (at least 1 hour).
Make the shiny chocolate glaze: warm more Heavy Cream and pour over chopped Dark Chocolate; stir until glossy. Stir in a little Unsalted Butter and Neutral Oil to create a pourable, shiny glaze. Cool slightly so it still pours but is not scorching.
Pour the glaze over the chilled cake for a smooth, mirror-like finish. Chill again until fully set, at least 30–60 minutes.
Trim the edges cleanly with a sharp knife (warm the knife under hot water and wipe dry between cuts for neat slices). Serve the Opera cake at cool room temperature so the buttercream is tender and the layers are distinguishable.
