Stir together 2 tablespoons water and rum in a small bowl. Sprinkle gelatin over mixture. Stir mixture, and set aside.
Combine sugar and cornstarch in a heavy saucepan; gradually whisk in milk and egg yolks. Bring to a boil over medium heat, whisking constantly; boil 1 minute. Stir in gelatin mixture until dissolved.
Stir together 1 cup custard mixture and morsels until smooth. Pour into Gingersnap Crust. Chill 30 minutes or until set. Set aside remaining custard mixture.
Beat whipping cream at high speed with an electric mixer until foamy; gradually add powdered sugar, beating until soft peaks form.
Fold 1 cup whipped cream into remaining custard mixture. Spoon over chocolate mixture. Chill pie and remaining whipped cream 2 hours or until pie is set. Spread remaining whipped cream over pie. Garnish, if desired.