Preheat the oven to 160 Celsius (320 Fahrenheit) and line two baking trays with baking paper.
Place 115g of the pistachios in the bowl of a food processor and process until finely ground. Just keep an eye on it as you process it to make sure you don't turn it into pistachio butter.
Place the ground pistachios and 55g of the caster sugar and the flour in a bowl and stir to combine.
Place the egg whites into the bowl of an electric mixer and whisk until soft peaks form. Add the remaining caster sugar gradually and then fold the egg whites into the pistachio mixture and fold through until well combine. Place heaped teaspoons on the baking trays. Chop the remaining pistachios and sprinkle over the top of the cookies.
Bake for 12 minutes or until lightly golden and transfer to a wire rack to cool. Dust with icing sugar to serve.
Notes
Feel free to substitute the pistachios with whatever nut you like.