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.