Preheat the oven to 190°C (375°F/Gas 5). Lightly grease ten 125ml (4fl oz/½cup) friand tins.
Sift the icing sugar and flour into a bowl. Add the ground pistachios, butter, egg whites and vanilla and stir with a metal spoon until just combined.
Spoon the mixture into the prepared tins, place on a baking tray and bake for 15–20 minutes, or until a skewer inserted into the centre of a friand comes out clean. Leave in the tins for 5 minutes, then turn out onto a wire rack to cool.
Put the caster sugar and 60ml (2fl oz/¼cup) water in a small saucepan and stir over low heat until the sugar has dissolved. Increase the heat, then boil for 4 minutes, or until thick and syrupy. Remove from the heat and stir in the chopped pistachios, then, working quickly, spoon the mixture over the tops of the friands. Dust with icing sugar before serving.