For the Meringue Shell: Heat oven to 275ºF. Beat 4 egg whites until stiff, gradually add sugar and cream of tartar. Spoon around bottom and sides of greased 10" pie plate. Bake for 1 hour. Remove from oven, allow to cool.
For the Mousse: Melt 12 oz. semi-sweet chocolate morsels in double boiler, remove from heat. Add egg yolks and whole egg, add rum.
In separate bowl, beat egg whites until stiff. In separate bowl, beat 2 cups heavy cream until thick. Fold egg whites and heavy cream into chocolate mixture, spoon into cooled meringue shell. Refrigerate a few hours.