First mix together the pistachios, icing sugar and cinnamon.
Next, take a sheet of filo, covering the others with a clean tea cloth, and butter it generously, using a brush. Then sprinkle it with a sixth of the nut-and-sugar mixture.
Now place another sheet on top, butter it and sprinkle with the same amount. Repeat with a third sheet and more of the mixture, pressing down the layers. Now begin again, so you end up with two stacks. Next, take each stack, cut it into four equal-sized rectangles, then take each rectangle and cut it into three haphazard triangles.
Place half the triangles on the baking tray and bake them on the middle shelf for 10-12 minutes, or until they are crisp and golden brown. Then transfer the first batch to a wire rack and allow the tray to cool before baking the second. Allow this batch to cool, too. When the wafers are cold, store them in an airtight plastic box and sprinkle with a little icing sugar just before serving.