Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom and side of 8-inch round cake pan with baking spray with flour. Grease and flour bottom and sides of 11x7-inch glass baking dish (do not use baking spray). Make cake batter as directed on box. Pour 1 1/3 cups batter into 8-inch round pan and remaining batter into 11x7-inch baking dish.
Bake shiny 8-inch pan 23 to 28 minutes (dark or nonstick 8-inch pan 26 to 33 minutes) and 11x7-inch dish 34 to 38 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pan and baking dish to cooling racks. Cool completely, about 30 minutes.
Cut 8-inch round cake crosswise in half. Spread 2 tablespoons frosting over top of 1 cake piece. Top with second cake piece.
Cut wedge shape from each long side of 11x7-inch cake, so cake is 4 inches wide at one end and 7 inches wide at other end. On tray, place larger cake piece with 4-inch end at one end of tray. Place stacked rounded cake at 7-inch end of cake on tray. This cake is the kettle corn. Freeze 1 hour before frosting to reduce crumbs.
Spread desired amount of remaining frosting over entire cake. Peel 5 sections of licorice apart; arrange lengthwise on popcorn bag.
Place white chocolate bar on center of popcorn bag. Write "Happy Birthday" on bar with decorating icing. Press kettle corn onto top of cake. Store loosely covered.