Is adorning cakes with fondant —knowing that it is often not eaten and is thrown away— considered an extravagance for which one will be held accountable, whether when selling the cake or when adding a layer of Styrofoam under the cake and covering it with fondant at the customer's request?

If the dough is something edible that people discard, then there is no sin upon the one who adorns food with it; rather, the sin is upon the one who discards it. Some jurists have permitted benefiting from foodstuffs for purposes other than eating, such as washing hands with flour and bran, and using salt for purification. However, if it is possible to replace the dough with something else inedible for adorning food, that would be preferable.