Knead until smooth and allow to rest for about 10 minutes.
Shape the paste into small balls the size of a walnut, and using a teaspoon handle, make a small hole in each ball and and fill it with the combined pistachio nuts and caster sugar; close the hole over the filling and reshape to a round shape.
Roll the balls in icing sugar and place in small paper cups or on a serving plate.
Decorate the top of each ball with a peeled pistachio nut.