Put the white, milk and plain chocolate in separate microwave-proof or heatproof bowls and add a third of the butter to each. Melt all the chocolate, either one at a time in a microwave oven or by setting each bowl over a saucepan of very gently simmering water, stirring occasionally until smooth.
Spread the melted white chocolate over 4 of the cakes using a small palette knife and dust with a little cocoa powder.
Put 2 tablespoons of the melted milk and plain chocolate in separate piping bags fitted with writing nozzles. Spread the milk chocolate over 4 more of the cakes and pipe dots of plain chocolate over the milk chocolate.
Spread the plain chocolate over the 4 remaining cakes and scribble with lines of piped milk chocolate. Leave to set.