1) Pre- heat the oven to 160 C/ Gas Mark 3.
2) Place the cup cases inside canapés tins or small muffin tins.
3) Grind the pistachio nuts in a food processor remove and set aside.
4) Separate the eggs and place the 6 egg yolks in a mixing bowl with the sugar. Beat well until thick and pale. Stir in the potato flour, orange zest and ground pistachio nuts.
5) In a separate clean bowl, whisk the 4 egg whites with a pinch of salt until they reach soft peak stage.
6) Fold the egg whites into the pistachio mixture and spoon into the cup cases so that they are 2/3rds full. Sprinkle the reserved pistachio nuts on top.
7) Bake for about 30 minutes.
8) Transfer to a rack to cool.
To decorate: Dust with icing sugar.