Buttercream 1/8 Sheet Marble Cake

CAKE: SUGAR, ENRICHED BLEACHED WHEAT FLOUR (FLOUR, NIACIN, REDUCED IRON, THIAMINE, MONONITRATE, RIBOFLAVIN, FOLIC ACID), EGG WHITES, SOYBEAN OIL, EGGS, WATER, CONTAINS LESS THAN 2% OF THE FOLLOWING: COCOA ALKALI PROCESSED, LEAVENING (SODIUM ACID PYROPHOSPHATE, BAKING SODA, MONOCALCIUM PHOSPHATE), SKIM MILK, PROPYLENE GLYCOL MONO AND DIESTERS OF FATS AND FATTY ACIDS, WHEY (A MILK DERIVATIVE), SALT, WHEAT STARCH, MODIFIED CORN STARCH, MONO AND DIGLYCERIDES, DEXTROSE, NATURAL AND ARTIFICIAL FLAVOR (CONTAINS MILK INGREDIENTS) SODIUM CASEINATE (MILK DERIVATIVE) XANTHAN GUM, POLYSORBATE 60, SODIUM STEAROYL LACTYLATE, CORN SYRUP SOLIDS, SOY LECITHIN, MAY CONTAIN ARTIFICIAL COLOR (YELLOW 5 LAKE), ICING: SUGAR, WATER, VEGETABLE SHORTENING (PARTIALLY HYDROGENATED SOYBEAN, COTTONSEED AND PALM OILS, MONO AND DIGLYCERIDES, POLYSORBATE 60, SORBITAN TRISTEARATE), COCOA ALKALI PROCESSED, CORN SYRUP, CONTAINS LESS THAN 2% OF THE FOLLOWING: COCOA, SOYBEAN OIL, SALT, TO PRESERVE FRESHNESS (POTASSIUM SORBATE, SODIUM BENZOATE), CITRIC ACID, MONOGLYCERIDES, SOY LECITHIN, CHOCOLATE LIQUOR, NATURAL AND ARTIFICIAL FLAVOR. COLORED ICINGS MAY CONTAIN LESS THAN 2% OF THE FOLLOWING: GLYCERINE, SUGAR, FD&C COLORS RED 3 & 40, BLUE 1 AND YELLOW 5 & 6), SORBITOL, MODIFIED FOOD STARCH, SODIUM BENZOATE AND POTASSIUM SORBATE (AS PRESERVATIVES), VEGETABLE GUMS, SALT AND CITRIC ACID.