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Break the dark chocolate into pieces and melt in short blasts in the microwave, or in a heatproof bowl over a pan of just simmering water, stirring frequently. Pour it onto a tray lined with baking parchment or foil.
Now break the milk chocolate into pieces and melt in the same way. Repeat with the white chocolate, then drizzle them both over the dark chocolate before it sets.
Shake the tray gently to level the mixture then leave to set somewhere cool. Chop into shards.