Shell the pistachios and scatter half of them over the tray along with half of the chopped mango pieces (I just scattered them over the middle part of the tray as in the pictures so that the chocolate bark doesn't end up too thin).
Melt the dark chocolate in a dish by microwaving in 30-second spurts and stirring until smooth. (Alternatively, melt it in a 'bain-marie', or a glass bowl placed over a saucepan of boiling water, being careful not to let the bowl touch the water)
Pour the chocolate over the nuts and fruit, smoothing it over slightly with a knife.
Melt the white chocolate in the same way described above and drizzle it unevenly over the dark chocolate layer. Trail a knife through it for a marbled effect if you like.
Scatter the remaining pistachios and mango pieces evenly on top of the chocolate, pressing in a little if necessary to make sure they 'stick'.
Put the whole tray in the fridge for at least half an hour, then break up the bark into uneven pieces.
Package up in a pretty, baking-paper-lined box, tie up with a ribbon, label and give the gift to the lucky recipient!
Use really good quality chocolate for the best results.Don't eat all the mango slices while you're making this - they're seriously moreish!