Combine chocolate wafer crumbs and melted butter in a medium bowl; stir well. Firmly press crumb mixture evenly in bottom and up the sides of a 10 inch pie plate. Bake at 350° for 8 minutes. Set aside, and let cool.
Place unsweetened chocolate in the top of a double-boiler; bring water to a boil. Reduce heat to low; cook until chocolate melts, stirring occasionally. Set aside.
Combine 1/3 cup sugar and cornstarch in the top of a double boiler. Add egg yolks; beat at medium speed of an electric mixture until thick and lemon colored. Stir in 2 cups whipping cream. Place over boiling water. Reduce heat to low; cook, stirring constantly. 35 minutes or until the mixture coats the back of a metal spoon (do not boil). Remove from heat.
Combine 1 cup custard mixture (egg yolk mixture) and melted chocolate in a small bowl; stir in vanilla. Pour into prepared crust. Chill until set.
Sprinkle gelatin over the cold water; let stand 1 minute. Add the softened gelatin to the remaining custard mixture. Place in the top of a double boiler; cook, stirring constantly, until the gelatin dissolves. Pour into a large bowl. Chill 20 minutes or until the consistency of unbeaten egg white, stirring occasionally. Stir in rum.
Beat egg whites (at room temperature) and cream of tartar at high speed just until foamy. Gradually add 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form and the sugar dissolves (2 to 4 minutes). Fold beaten egg whites into the thickened gelatin mixture. Spoon over the chocolate layer in the crust. Chill 4 hours. Garnish, if desired.