Method

• 1.
  Preheat oven to 160°C/140°C fan-forced. Grease base and side of 3 x 20cm round cake pans and line with baking paper. In a stand mixer fitted with the paddle attachment, cream the butter, sugar and vanilla on medium-high speed for 10 minutes, scraping down side of bowl occasionally, or until pale and creamy in colour and texture. Add eggs, one at a time, mixing well between each addition until well incorporated.
• 2.
  Place the flour, cocoa, cornflour, baking powder, 1 tsp fine salt and bicarb in a large bowl and whisk to evenly combine. Add flour mixture to egg mixture, in three batches, mixing well on medium speed and scraping down side of bowl between each addition. Add sour cream and mix on low speed until just combined.
• 3.
  Divide batter equally among prepared pans and use a spatula to level tops. Bake for 35-40 minutes until a skewer inserted in the centres comes out clean. Place a large piece of baking paper on a large wire rack. Remove cakes from pans and invert onto lined rack so flat side is facing up. Cool completely.
• 4.
  To make the chocolate buttercream, add chocolate and icing sugar to a medium heatproof bowl. Bring the cream to a simmer in a small saucepan over medium-high heat, then pour over the chocolate mixture. Stand for 5 minutes, then whisk until smooth. Whisk in the 2 egg yolks, then the 30g butter. Meanwhile, in a stand mixer fitted with the paddle attachment, beat extra 360g butter and cocoa on medium-high speed for 5 minutes or until smooth and creamy.
• 5.
  With the motor running on low, gradually add melted chocolate mixture until fully incorporated, scraping down side of the bowl as you go. Set aside. Next, in a stand mixer fitted with the whisk attachment, whisk the extra 7 egg yolks on medium-high speed until very pale and creamy. While the eggs whisk, place the caster sugar and 100ml water in a small saucepan over medium heat and cook, stirring constantly, until sugar dissolves, then simmer until mixture reaches 116°C on a sugar thermometer (about 6-8 minutes).
• 6.
  With mixer still running with the yolks, carefully and slowly drizzle the hot sugar mixture into the yolks, keeping clear of the side of bowl and the whisk, to ensure you get a good mixture and not too much mess around the bowl. Continue to whisk on medium-high speed for 10 minutes, until cooled, thickened and fluffy.
• 7.
  Reduce speed to medium and gradually add the chocolate butter mixture to the egg and sugar mixture, whisking until smooth and well incorporated. Use buttercream immediately (it will become very hard to spread otherwise).
• 8.
  Working quickly, place one cake on a serving plate and spread top with 3/4 cup of the buttercream, leaving a 1cm border around edge of cake. Repeat layers twice more, then use remaining buttercream to fill gaps between cakes on the side. Finish by quickly spreading buttercream all over side and top of cake, using a small offset spatula to create a rough, sweeping effect. Sprinkle top immediately with cacao nibs.
• 9.
  Serve, or store (see note).