Method

• 1.
  Preheat oven to 180°C. Trace a 20cm x 30cm lamington pan outline onto 3 sheets of baking paper. Grease 3 baking trays and place a prepared sheet of baking paper, tracing-side down, on each tray.

• 2.
  Place eggwhites and icing sugar in a stand mixer fitted with the whisk attachment and whisk for 8 minutes or until stiff peaks form. Add hazelnut meal, flour, cocoa and orange zest, and whisk briefly to combine. Fold through egg yolks and divide among prepared trays, smoothing out with a spatula to fit the outlined rectangle. Bake, swapping trays every 10 minutes, for 30 minutes or until firm to the touch. Set aside to cool completely.
• 3.
  For the whipped chocolate ganache, place cream mixture and vanilla in a saucepan and bring to the boil. Place chocolate in a heatproof bowl, strain over hot cream and stand for 5 minutes. Stir until smooth, then chill for 20 minutes to cool to room temperature. Transfer to a stand mixer fitted with the whisk attachment and whisk until soft peaks form. Chill until needed (stand at room temperature for 10 minutes before assembling to soften slightly).
• 4.
  For the orange buttercream, place butter in a stand mixer fitted with the paddle attachment and beat until pale. Gradually add icing sugar and orange zest and juice, and beat constantly until combined. Cover with plastic wrap and chill until needed (stand at room temperature for 10 minutes before assembling to soften slightly).
• 5.
  For the chocolate glaze, soak gelatine in cold water for 5 minutes to soften. Place cream, sugar and 1/2 cup (125ml) water in a saucepan over medium-high heat and bring to the boil. Squeeze excess water from gelatine, then add leaves to cream mixture and stir until dissolved. Remove from heat, whisk in cocoa, strain into a jug and set aside to cool to room temperature.
• 6.
  To assemble, grease base and long sides of a 20cm x 30cm lamington pan and line with 3 layers of baking paper, allowing paper to extend 3cm over long sides (this makes it easier to remove cake from pan). Use the base of the lamington pan as an outline to trim sponges into 3 rectangles.
• 7.
  Line base of prepared pan with 1 sponge. Spoon over whipped ganache and smooth surface with a palette knife. Top with second sponge, then buttercream, smoothing with a palette knife. Top with remaining sponge and pour over glaze to coat. Chill for 2 hours or until set.
• 8.
  Using a knife dipped in boiling water and dried, cut opera cake into sixteen 10cm x 4cm pieces. Scatter with freezedried mandarin, if using, to serve.