Add coffee powder to cake mix, and bake according to directions on package in 13x9" pan. While cake bakes, combine water, sugar, butter and rum in a small pan. Heat until sugar is dissolved and buttermelted. Spoon over hot cake until all liquid is absorbed. Cool cake completely before frosting. Combine frosting mix, coffee powder, and whipping cream in mixing bowl. Chill 15" in refrigerator. Beat until thickened. Top cake with mixture and garnish with shaved Hershey bar.