Does anyone have or know of a frosting recipe that would glow under a black light? I heard that putting tonic water in the mix works but it also changest he flavor. This girl is having a sweet 16 black light party and wants her cake to glow multiple colors.
In order to get a cake to actually "glow" under a blacklight, you'd have to use some not-so-safe ingredients. However, a bright white will show up great under a blacklight.
You may have to go with LED lights if she insists on the different colors. Ah, the younger generation - they think we can do anything!
The ingredient in tonic water(quinine) will make your frosting glow but in large amounts it can harm you. If she wants a cake that glows I would frost the cake as posted above and only coat the decorations in tonic water. Just make sure you tell them to remove the decorations before serving and not to eat them. Small amounts won't kill anyone it's just better to be safe than sorry you never know if someone might have an allergic reaction. http://www.drugs.com/sfx/quinine-side-effects.html
You will have to tell the birthday girl that this would NOT be edible at all. The best you can do is carve some plastic that shows up well under black light, and add pieces around the base or between the tiers of the cake. You might be able to find different colours like yellow, green, blue.
The quinine dissolved in tonic water shows blue under black light (=UV). But I'm afraid that it might be less visible if you painted it onto dried royal icing or gumpaste. Paint enough of any fluorescent chemical on to see it, and the decorations will definitely taste bitter if the stuff is edible to begin with.
Now you could run the tonic water in a cake fountain, that would look pretty cool, but you only get one colour.
So where the heck do some people get their ideas for cakes anyway? Have the TV shows given people the false impression that you can make anything into an edible cake? I watch Cake Boss and I appreciate the skill required to distill a customer's idea into cake reality. Remember the cake with "blood" for the vampire party? No I don't watch any other cake shows...that one is more than enough.