In heavy saucepan combine 4 T sugar, cornstarch, and mint-chocolate chips.
Gradually stir in milk.
Cook, stirring constantly, over medium heat until mixute boils.
Boil 1 minute; remove from heat.
Transfer to a large bowl; cover surface of chocolate mixture with plastic wrap.
Cool to room temperature.
In a medium bowl combine heavy cream with remaining 2 T sugar; beat until stiff.
Remove plastic wrap from chocolate; beat well.
Fold in whipped cream.
Spoon into crust.
Chill 2 - 3 hours until firm.