Melt both chocolates and the milk in a double boiler over low heat. Remove from the heat and add in the corn syrup, chocolate-flavoured liqueur and vanilla extract. Mix together and allow to cool completely.
With an electric mixer on high speed, beat the meringue powder with 6 tablespoons water in a medium bowl until glossy peaks form, 2 to 3 minutes.
With a rubber spatula, gently fold the beaten meringue and the whipped topping into the cooled chocolate mixture until no streaks of white remain. Spoon into the prepared crust and spread evenly. Cover with plastic wrap and freeze until frozen, for a couple of hours. Garnish with grated chocolate and serve.