Combine 1 cup sugar, gelatin, and salt in a medium saucepan; add milk and egg yolks, stirring until well blended. Cook over medium heat, stirring constantly, 5 minutes or until slightly thickened. Remove from heat, and add chocolate morsels; stir until chocolate melts. Add rum and vanilla; chill until mixture is the consistency of unbeaten egg whites.
Beat egg whites (at room temperature) until foamy; gradually add remaining 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold beaten egg whites into chilled chocolate mixture.
Spoon two-thirds of chocolate mixture into pastry shell; cover with 1/2 cup whipped cream. Dollop remaining chocolate mixture onto whipped cream; swirl chocolate and whipped cream together. Chill until set. Garnish with additional whipped cream, if desired.