Cake

Buttercream frosting is almost always safe, but some states and health depts still don’t allow it. Usually this is because they prohibit all uncooked dairy products in cottage foods, even though butter is perfectly safe when left unrefrigerated. So in this instance, I’d say it’s not a matter of whether or not it is perishable, but rather that it falls under the blanket “dairy” prohibition and is then not allowed. If it isn’t specifically listed as allowed, you need to contact your health or ag dept to determine if they will allow it.