Preheat oven to 180°C/160°C fan forced. Grease the base and side of two 20cm and two 15cm (base measurement) round cake pans and line with baking paper.
Place 500g butter and 400g chocolate in a large saucepan over low heat. Cook, stirring, for 5 minutes or until melted and smooth. Pour into a large bowl. Set aside for 10 minutes to cool.
Whisk 4 eggs, 235g (1 cup) sour cream and 125ml (1/2 cup) milk in a bowl until smooth. Whisk into the cooled chocolate mixture until smooth. Add 530g (21/2 cups) sugar, 200g (11/3 cups) plain flour, 100g (2/3 cup) self-raising flour, 95g (1 cup) cocoa powder and 1/2 teaspoon bicarb. Mix well until smooth. Divide the mixture between one 20cm and one 15cm prepared pan and smooth the surface. Bake the smaller cake for 1 hour 15 minutes and the larger cake for 1 hour 30 minutes or until a skewer inserted in the centre of each cake comes out clean. Set aside in the pans to cool completely.
Repeat with the remaining butter, chocolate, eggs, sour cream, milk, sugar, plain flour, self-raising flour, cocoa powder and bicarb to make another 20cm and 15cm cake.
For the meringue buttercream, place the egg whites and sugar in a heatproof bowl over a saucepan of simmering water, making sure the base of the bowl doesn’t touch the water. Use a balloon whisk to stir for 5-8 minutes or until the sugar dissolves and the mixture reaches 70°C on a cook’s thermometer. Transfer to a clean bowl. Use electric beaters to beat for 5 minutes or until firm peaks form and the mixture is almost at room temperature. Add the butter, a few pieces at a time, beating well after each addition, until the mixture is thick and smooth. Beat in the vanilla. Cover and set aside.
Use a large, sharp knife to trim the tops of the cakes to flatten. Carefully cut each cake in half horizontally. Spread a thin layer of jam over 3 larger cake layers and 3 smaller cake layers. Leave the other 2 cake layers plain.
For the marshmallow, stir the sugar and hot water in a saucepan over low heat, brushing down the sides of the pan with a wet pastry brush, for 3 minutes or until the sugar dissolves. Bring to the boil. Simmer rapidly, without stirring, for 2 minutes. Set aside for 2 minutes to cool slightly. Place 80ml (1/3 cup) cold water in a small bowl. Sprinkle with the gelatine and whisk until combined. Pour the gelatine mixture into the warm sugar syrup and stir until the gelatine dissolves and the mixture is clear. Set aside for 20 minutes to cool to room temperature. Transfer to a large bowl. Use electric beaters to beat for 4-5 minutes or until the mixture is thick and leaves a ribbon trail when the beaters are lifted. Beat in the vanilla and enough food colouring to tint the mixture pink. Working quickly, spread a thin layer of marshmallow over the jam-topped cake layers. Stack the three larger cake layers on top of each other and the three smaller cake layers on top of each other, then top each with a plain cake layer. Stand for 15 minutes or until the marshmallow is set.
Coat both cake stacks with a thin layer of buttercream, filling any gaps and smoothing the surface as much as possible. Place in the fridge for 30 minutes or until just set. Spread with another slightly thicker layer of buttercream and use a spatula or cake scraper to smooth the surface. Place in the fridge for 30 minutes or until just set.
Line a tray with baking paper. Place the white chocolate in a microwave-safe bowl and heat on Medium, stirring every 30 seconds, until melted and smooth. Divide into 2 portions. Tint 1 portion pink and leave the other portion white. Dip the back of a teaspoon into one of the chocolates and smear onto the lined tray to create a long shard. Repeat with the remaining chocolate. Set aside for 5 minutes to set.
To assemble, reserve 7 Royals and cut the remaining biscuits in half. Place the larger cake stack on a serving board. Top with the smaller cake stack. Press the halved Royals around the base of each cake layer. Place the remaining buttercream in a piping bag with a 5mm plain nozzle. Pipe dots over the side of the smaller cake stack and in between the Royals on each stack. Decorate with the reserved Royals, chocolate shards and silver cachous.