Combine chocolate and cream in a small saucepan, stir over low heat until melted and smooth (4-5 minutes), stir through orange-blossom water and 50gm pistachios. Pour into a shallow tray and refrigerate until firm (2-3 hours). Finely chop remaining pistachios.
Using a dessert spoon dipped in hot water, scrape ganache into large curls, dust with cocoa, top with a pinch of finely chopped pistachio and serve. Truffles will keep refrigerated in an airtight container for 10 days.