Prepare cake per directions on package for two 9-inch rounds but fold in the orange rind. Bake and cool completely. When cool, using sharp knife cut in half horizontally to make 4 layers. Sprinkle each layer with rum. Place first layer on cake plate( cut side down) and spread with 1/3 of the filling.Repeat with two more layers. Top layer should be top side up. Frost with chocolate frosting and if desired press 1 cup chopped walnuts around the sides of cake. Refrigerate overnight.
Filling: In small bowl, at medium speed, whip the cream with sugar, salt until stiff peaks form. Fold in vanilla.
Frosting: In medium bowl, with mixer at low speed, beat the chocolate (melted)sugar and water until well blended. Add eggs, one at a time, beating well after each addition. Add butter and beat until smooth.