1Heat oven to 350°F (325°F for dark or nonstick pans). Grease bottoms and sides of 2 (9-inch) round cake pans with shortening; lightly flour.
2Make cake mix as directed on box, using oil and eggs and substituting carbonated beverage for the water. Bake and cool as directed.
3In large bowl, beat butter with electric mixer on low speed 1 to 2 minutes or until creamy. Gradually add powdered sugar, 1 cup at a time, beating after each addition. Add rum and rum extract. Continue beating until frosting is smooth and spreadable.
4Place 1 cake layer, rounded side down, on serving plate. Spread with a layer of frosting. Top with second cake layer, rounded side down. Frost side and top of cake with remaining frosting. Decorate as desired.