Mhays wrote: Dry ice smoke would be perfect, but would probably cause the cake to be soggy, and it's a pain to deal with
Dry Ice smoke is denser than air, therefore it will sink and not give you the effect you are looking for, plus it's toxic if you breathe a lot of it. Check with a model train store. As a kid, I remember getting these little white tablets that you would put in the chimney of a certain kind of locomotive (made by Lionel) that would emit puffs of smoke. I'm not sure how they worked, but I think it involved heating them up. You could aways go low tech and light a cigarette and stick it down inside the hole.
Steve Z.
“Only the pure in heart can make a good soup.”
― Ludwig van Beethoven