In small saucepan over low heat, melt chocolate, stirring constantly until smooth. With small paintbrush, spread 1 teaspoon chocolate thickly and evenly over inside of each baking cup. Turn cups upside down on cookie sheet. Refrigerate 20 minutes or until set.
Meanwhile, in medium bowl, combine white baking bar and butter. In small saucepan, heat cream until very hot. (Do not boil.) Pour hot cream over baking bar mixture; beat until mixture is melted and smooth. Beat in powdered sugar and rum extract until well blended. Cover; refrigerate about 45 minutes or until mixture is thick and mounds when dropped from spoon.
Spoon mixture into chocolate cups, or pipe mixture from decorating bag or plastic bag with one corner snipped off. Refrigerate 15 minutes or until firm. Before serving, peel foil cups from truffles. Store in refrigerator.