Heat oven to 190C/170C fan/gas 5. Liberally grease 16 madeleine moulds (or you could use tartlet tins), then sift over a light dusting of flour.
Tip the pistachios into a mini chopper or food processor and grind until fairly fine but with a bit of texture. Chop the orange peel finely (not necessary if you are using ready-cut).
Mix the flour, pistachios, orange peel and sugar in a large bowl. Put the melted butter, egg yolk and orange blossom water in a separate bowl and mix together with a fork.
Whisk the 2 egg whites in a clean bowl until they form soft peaks. Fold the butter mixture into the dry ingredients, then fold in the egg whites using the whisk blades.
Carefully spoon the mixture into the prepared moulds and bake for 12-15 mins until golden and firm to the touch. Cool for a few mins in the tin, then turn out onto a wire rack and leave to cool completely. Dust with icing sugar to serve. Best eaten on the day.