1.Heat cream in a small saucepan until almost boiling. Add chocolate; remove from heat. Stir until smooth. Cool to room temperature, about 2 hours, stirring occasionally. Transfer mixture to a small bowl of an electric mixer; beat ganache for 3 minutes or until fluffy and lighter in colour – do not overbeat or mixture will split. Refrigerate for 1 hour or until thickened and spreadable.
2.Line a large oven tray with baking paper. Melt chocolate and butter in a small, heavy-based saucepan over low heat. Sift icing sugar into a large bowl; stir in Rice Bubbles, coconut and chocolate mixture until combined. Press the mixture onto tray until 1cm thick. Refrigerate for 1 hour or until firm. Break half of the crackle slab into large shards for the top of the cake. Coarsely crumble the remaining crackle.
3.Meanwhile, preheat oven to 180°C (160°C fan-forced). Grease two deep 20cm round cake pans; line bases and sides with baking paper.
4.Stir cocoa and the boiling water in a small heatproof jug until smooth. Cool.
5.Beat butter, extract and sugar in a medium bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time, then cocoa mixture and melted chocolate until combined (mixture may look curdled at this stage). Beat in flours and buttermilk, in two batches, on low speed, until smooth. Divide mixture between pans; smooth the surface.
6.Bake cakes for 40 minutes or until a skewer inserted into the centre comes out clean. Leave cakes in pans for 5 minutes before turning, top-side up, onto wire racks to cool.
7.Split cakes in half. Place one cake layer on a plate or stand. Spread with one-fifth of the ganache, then a third of the crumbled crackle; sprinkle with a third of the Milo. Repeat layering, finishing with a ganache layer. Spread remaining ganache around the side. Top with large crackle shards.
Undecorated cakes suitable to freeze. Not suitable to microwave.Note