Step 1

Add the chocolate to a large bowl set over a pan of barely simmering water. Stir until the chocolate is melted. Remove from the heat and stir in the sour cream. Refrigerate for 1 hour or until the mixture has thickened and is of a spreadable consistency.

Step 2

Line a large oven tray with baking paper. Melt the 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.

Step 3

Preheat the oven to 180°C (160°C fan-forced). Grease two deep 20cm round cake pans; line the bases and sides with baking paper.

Step 4

Whisk the cocoa and the water in a small heatproof jug until smooth. Cool for 10 minutes.

Step 5

Beat the butter, vanilla 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 the mixture between the pans; smooth the surface.

Step 6

Bake cakes for 40 minutes or until a skewer inserted into the centre comes out clean. Leave cakes in the pans for 5 minutes before turning, top-side up, onto wire racks to cool.

Step 7

Split cakes in half. Place one base 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 the layering, finishing with a ganache layer. Spread the remaining ganache around the side. Top the cake with large crackle shards.