Roast the pistachios on baking sheet for about 10 minutes. Tip into a bowl to cool down a little, then put them in a mini food processor* (the type that comes with a hand blender or a separate mini chopper) with 40g caster sugar and blitz to a paste. This will take up to around 10 minutes. At first it will just seem dry, but keep going until it turns to a moist, dark brown smooth paste.
Put the cream and milk in a large saucepan and heat until hot, but just before it boils (when small bubbles start to appear), then turn off the heat.
Beat together the eggs and sugar until thick. Pour the hot cream over the whisked yolks and sugar, while whisking, then return to the pan and place over a low to medium heat. Stir constantly with a wooden spoon until the custard thickens and it leaves a clean mark when you coat the back of the spoon and drag your finger through it (this will take about 5-8 minutes).
Turn off the heat and stir in the pistachio paste, and then pour through a fine sieve back into the bowl, pushing the paste through the sieve with the back of a spoon. The natural colour is brown, rather than green. If you want green ice cream, stir in a little green food colouring**.
Pour into a freezer proof plastic box and press a piece of cling film against the surface to prevent a skin from forming while it cools.
When cool, put the lid on and freeze for at least 4 hours, until firm.
Remove the ice cream from the freezer. Turn it out of the container, cut it into chunks with a sharp knife and put them in a food processor. (To help it out, pour hot water of the bottom of the box for a few seconds – it should then slip out, if not, ease around the sides with a knife. If it is rock solid and impossible to cut, leave it to soften for a few minutes). Blitz in the food processor for 2-3 minutes until smooth and creamy. Spoon back into the box, cover and freeze again until hard, then repeat this process twice more. It can be left overnight before blitzing in the food processor.
Return to the freezer and remove for 10-15 minutes before serving, to soften a little.