Reserve a quarter of the pistachios and finely grind the remainder in a food processor. Tip into a pan with the milk and sugar, and bring gently to the boil, stirring to dissolve the sugar. Remove from the heat, leave to cool, then chill.
Finely chop the reserved pistachios. Whip the cream to soft peaks. Fold in the custard, then the nut milk until evenly mixed. Churn in an ice cream machine or put the bowl in the freezer until the mixture is set 5cm in from the edges. Remove and whisk to break down the large ice crystals. Freeze and whisk again, then transfer to a rigid container and scatter with the chopped pistachios. Freeze completely until firm.
Half an hour before serving, transfer the ice cream to the fridge to soften.