Bring the water, sugar and lemon juice to the boil in a pan, reduce the heat, simmer for approx. 10 mins., remove the pan from the heat, puree the pistachios with the sugar syrup, leave to cool completely. Fold in the cream, place in a wide stainless steel bowl, freeze the mixture for approx. 4 hrs., stirring 3 times during that time.
Boil the sugar and water in a wide-bottomed pan without stirring. Reduce the heat and simmer, shaking the pan occasionally until a light brown caramel has formed. Add the butter and pistachios, mix. Spread out immediately on a sheet of baking paper, cool and break into pieces.