Prepare the cake mixas the label directs for a 9-by-13-inch cake, bake and cool. Cut in half crosswise. Spread frosting on one half, then top with second half. Cover the layer cake with frosting.
Dust a flat surface with cornstarch. Using a rolling pin, roll out the white fondant to at lease 16 by 18 inches large (about 1/8 inch thick).
Drape the white fondant over the cake, covering the cake but leaving one short end exposed with a slight overhang, as shown. Smooth the fondant, then cut away excess, except for the overhang.
Roll out the red fondant on a cornstarch-dusted surface to 1/16 inch thick. Cut into about 15 strips that are 14 by 1/2 inch each.
One at a time, spread some water on the back of each red fondant strip with your finger and attach to the cake, as shown, trimming the strips as needed. (Do not use too much water, or red dye will bleed.)
Press some of the caramel corn onto the uncovered side of the cake. Continue adding Caramel corn until it looks like the bag is overflowing, using more frosting to stick the pieces together.