Cookies and cream marble cake with buttercream icing

Buttercream icing (powdered sugar [sugar, cornstarch], vegetable shortening [palm oil, soybean oil, cottonseed oil, monoglycerides, polysorbate 60], nonfat dry milk, dried egg whites, natural and artificial flavor, salt), sugar, enriched wheat flour (bleached wheat flour, niacin, reduced iron, thiamine mononitrate, riboflavin, folic acid), eggs, chocolate & vanilla cake concentrate (bleached wheat flour, cocoa powder [processed with alkali], nonfat dry milk, palm oil, modified food starch [corn], sugar, potassium bicarbonate, dextrose, sodium acid pyrophosphate, salt, soy lecithin, soybean oil, whey, color [yellow corn meal, spices {turmeric, annatto}], gums [cellulose, guar, xanthan], sodium stearoyl lactylate, cornstarch, natural and artificial flavors, monocalcium phosphate, cinnamon), soybean oil, butter, egg whites (egg whites, guar gum, triethyl citrate), dark chocolate cookie grind (wheat flour, sugar, soybean oil, palm oil, cocoa [processed with alkali], invert sugar, contains 2% or less of the following: corn flour, soy lecithin, chocolate liquor, cornstarch, salt, baking soda, natural flavor), chocolate sandwich cookie (wheat flour, sugar, vegetable shortening [soybean oil, hydrogenated soybean oil], cocoa [processed with alkali], contains 2% or less of the following: cornstarch, molasses, sodium bicarbonate, salt, soy lecithin, natural and artificial flavors, ammonium bicarbonate, propylene glycol, ethyl alcohol, caramel color [sulfites]), soy lecithin, emulsifier (water, mono - and diglycerides, sugar).