Step 1

Preheat oven to 180°C. Grease two deep 20cm round cake pans; line bases and sides with baking paper.

Step 2

Sift cocoa into a medium bowl; stir in the hot water until smooth. Cool. Sift flours, soda and sugar into a large bowl of an electric mixer. Add buttermilk, eggs, oil and cocoa mixture. Beat on low speed until just combined. Divide mixture between pans.

Step 3

Bake cakes for 50 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.

Step 4

Meanwhile, make white chocolate buttercream; beat butter and sifted icing sugar in a small bowl with an electric mixer until light and fluffy. Gradually beat in chocolate. Make dark chocolate buttercream; beat butter and sifted icing sugar in a small bowl with an electric mixer until light and fluffy. Gradually beat in chocolate. Make milk chocolate buttercream; beat butter and sifted icing sugar in a small bowl with an electric mixer until light and fluffy. Gradually beat in chocolate. Keep buttercreams in a cool place until ready to use.

Step 5

Trim tops of cakes to level. Split cakes in half. Place one cake layer on a cake stand or plate. Reserve half the white chocolate buttercream for top and side of the cake (in step 6). Spread cake layer with a third of the remaining white chocolate buttercream; top with another cake layer. Repeat layering with remaining thirds of white buttercream and cakes, finishing with a cake layer. Refrigerate for 30 minutes.

Step 6

Spread dark chocolate buttercream around lower third of the cake. Spread milk chocolate buttercream around middle third of the cake. Spread half the reserved white chocolate buttercream around top third of the cake; spread remaining reserved white chocolate buttercream on top of the cake. Use a large palette knife to smooth side and blend colours slightly. Dust cake with extra cocoa powder.