Pulse chocolate wafers in a food processor 8 to 10 times or until finely crushed. Stir together wafer crumbs and butter, and firmly press mixture on bottom, up sides, and onto lip of a lightly greased 9-inch pie plate. Freeze crust 30 minutes.
Whisk together 2/3 cup sugar and 3 Tbsp. cornstarch in a 3-qt. heavy saucepan.
Whisk together egg yolks and milk in a small bowl; whisk yolk mixture into sugar mixture in pan, and cook over medium heat, whisking constantly, 10 to 12 minutes or until mixture thickens. Cook 1 more minute. Remove from heat.
Microwave chocolate in a microwave-safe glass bowl at HIGH 1 1/2 minutes or until melted, stirring at 30-second intervals. Whisk melted chocolate, rum, and vanilla into thickened filling. Spoon filling into prepared crust. Place plastic wrap directly onto filling (to prevent a film from forming), and chill 8 to 24 hours.
Beat whipping cream and 1/4 cup sugar at medium-high speed with an electric mixer until soft peaks form. Top pie with whipped cream, and garnish, if desired.
Note: We tested with Nabisco FAMOUS Chocolate Wafers and Ghirardelli 60% Cacao Bittersweet Chocolate Baking Bars.