Rub the skins from the soaked pistachios and discard.
Puree the pistachios and lime juice with a hand held blender or in a food processor until smooth. Be patient, it can take a little while. Once smooth, add in the avocado flesh, maple syrup and vanilla and then blend again until smooth.
Blend in the milk sparingly until it is a nice mousse-like consistency.
Pour into a shallow container and freeze, stirring occassionally.
Remove from the freezer ten minutes before serving.