In a heavy saucepan, combine 1/2 cup sugar, Knox gelatin, and salt. Stir in milk and egg yolks. Cook and stir over low heat until slightly thickened. Remove from the heat and add chocolate chips. Stir until chips are melted. Add rum. Chill until partially set. Beat the egg whites until soft peaks form. Gradually add 1/4 cup sugar, beating until stiff peaks form. Fold into chocolate mixture. Beat whipping cream with vanilla and desired sugar, until peaks form. Layer chocolate mixture and then whipping cream mixture. Swirl top to marble. Chill until firm.