Finely chop the pistachio nuts and save the dust. Pound the dust to a smooth paste with a little water and green colour to moisten.
Add this paste to the yolks and whisk until thick and an even pale green, about 5 minutes.
Meanwhile bring to the boil the sugar and the water. While still bubbling, trickle onto the yolks and whisk at a high speed until thickened and cold. Place in