In a large mixing bowl, mix together pudding mix and cold milk at low speed for 2 minutes, until it begins to thicken slightly. In a small mixing bowl, use an electric mixer to beat together cream cheese, butter, and powdered sugar until it is the consistency of thick frosting. Add cream cheese mixture to the pudding mixture and beat together on medium-high speed until smooth. Gently fold in (or mix on low speed) the whipped topping, until the mixture is uniformly pale green with no white streaks. Pour filling into prepared pie crust. Allow to chill for at least 2 hours before serving. Optional: Garnish pie by piping whipped cream around the edges and/or sprinkling with chopped pistachios.