In large saucepan set over medium-low heat, heat marshmallows and milk; cook, stirring, for 5 to 7 minutes or until melted. Let cool. Stir in mint extract.
Using electric mixer, whip cream until stiff peaks form; fold into marshmallow mixture. Fold in half of the chocolate chips.
Spoon filling into chocolate pie crust. Refrigerate for 4 to 8 hours or until set.
Heat remaining chocolate chips in heatproof bowl set over saucepan of barely simmering water for 5 to 7 minutes, stirring, until chocolate is melted. Let cool. Drizzle chocolate over pie; refrigerate until chocolate is set.