Into a food processor break crackers and finely grind.
With motor running, add butter.
Press crumb mixture onto bottom and up side of a 9 1/2-inch (5-cup) pie plate and bake in middle of oven 6 to 8 minutes, or until shell is a shade darker.
Cool shell on a rack.
In a shallow baking pan toast pistachios in middle of oven 5 minutes, or until a shade darker.
Reserve 1 1/2 tablespoons pistachios for sprinkling on pie and finely chop remainder.
In a saucepan dissolve sugar in milk over moderate heat, stirring, and remove pan from heat.
In a metal bowl whisk milk mixture and vanilla into halvah until halvah is dissolved.
Set bowl with halvah mixture in a larger bowl of ice and cold water and cool mixture, stirring occasionally.
In a bowl beat cream until it just holds stiff peaks.
Fold cream and chopped pistachios into halvah mixture and pour into shell.
Freeze pie, uncovered, 1 hour.
Sprinkle reserved pistachios over pie.
Freeze pie, uncovered, until frozen hard, about 4 hours.
Pie may be made 2 days ahead and frozen, covered with plastic wrap and foil after 5 hours.
Let pie stand at room temperature about 15 minutes to slightly soften (for ease of cutting) before serving.