Method

• 1.
  Preheat oven to 180°C/160°C fan-forced. Grease base and side of a 22.5cm springform pan and line base with baking paper.
• 2.
  For the cake, place the egg whites and sugar in a stand mixer fitted with the whisk attachment. Whisk on high speed to a firm meringue (by adding the sugar all at once, you get a denser meringue, which adds to the texture of the sponge). Add yolks and whisk until just combined.
• 3.
  Remove bowl from the mixer and sift over cocoa powder. Using a spatula, gently fold cocoa through. Spoon into prepared pan and spread evenly (the sponge batter will appear very voluminous, but will shrink after baking). Bake for 14 minutes or until sponge springs back when pressed. Set aside to cool completely in pan on a wire rack. Run a small sharp knife around the edge of the sponge and unmould. Transfer sponge to a board. Using the base of a 20cm springform pan as a guide, cut out a circle of cooled sponge for assembly.
• 4.
  For the dark chocolate mousse, using a stand mixer fitted with the whisk attachment (or a hand mixer), whip the cream to medium firm peaks, and chill until required. Melt chocolate in a large heatproof bowl over a saucepan of simmering water (don’t let base of bowl touch the water). Set aside.
• 5.
  Place milk in a small saucepan and bring to the boil over high heat. Meanwhile, whisk yolk and sugar by hand in a large heatproof bowl until pale, creamy and doubled in volume. Slowly whisk in one-third of the milk until smooth. Whisk in remaining milk, then pour mixture back into pan. Cook, stirring in a figure 8, over medium heat, for 3-4 minutes, until mixture coats the back of a spoon. Pour through a fine sieve onto the melted chocolate and whisk until just combined. Cover and set aside at room temperature to cool down to body temperature, then whisk in one-third of the chilled whipped cream until smooth. Gently fold in remaining cream until thoroughly combined. Set aside.
• 6.
  For the milk chocolate mousse, using a stand mixer fitted with the whisk attachment (or a hand mixer), whip 1 cup (250ml) cream to medium firm peaks. Chill until required.
• 7.
  Melt the chocolate in a large heatproof bowl over a saucepan of simmering water (don’t let base of bowl touch the water). Set aside.
• 8.
  Place milk and extra 1/4 cup (60ml) cream in a small saucepan. Bring to the boil over high heat. Meanwhile, whisk yolk and sugar by hand in a large heatproof bowl until pale, creamy and doubled in volume. Slowly whisk in one-third of milk mixture until smooth. Whisk in remaining milk mixture, then pour mixture back into pan. Cook, stirring in a figure 8, over medium heat, for 2-3 minutes, until mixture coats the back of a spoon.
• 9.
  Drain gelatine and squeeze out excess water. Add to pan and stir until dissolved. Pour through a fine sieve onto melted chocolate and whisk to combine. Cover and set aside to cool to body temperature. Whisk in one-third of the chilled whipped cream until smooth. Gently fold in remaining cream until thoroughly combined. Set aside.
• 10.
  To assemble, grease side of a 20cm springform pan and line side with baking paper. Position the sponge on a plate as the base and place pan over top. Spoon dark chocolate mousse over sponge and level surface with a spatula. Freeze for 30 minutes or until mousse has set (the longer it sets, the more defined the layers will be). Repeat this process with milk chocolate mousse. Freeze for 30 minutes.
• 11.
  Meanwhile, for the glaze, place sugar, cream, cocoa and 1/2 cup (125ml) water in a medium saucepan. Bring to the boil over high heat, whisking until smooth. Reduce heat to medium and simmer for 5 minutes, stirring occasionally. Remove from heat. Drain and squeeze out excess water from gelatine, then add to pan with chocolate. Stir until smooth. Strain through a fine sieve into a jug. Set aside to cool for 5 minutes. Pour over cake. Chill for 30 minutes to set.
• 12.
  To serve, unmould cake and peel away baking paper. Cut using a hot knife.