So you are saying the cake will be under a black light? Would white frosting glow or white MMF?
ok, I'm not sure if you will find these helpful but I did a google search for UV reactive materials and foods and this is what I got, not may of them are edible and the ones that are don't go well with cake, like spinach.
I also found this instruction to make glowing jello
step 1Assemble Ingredients
You will need:
Jello packet (or no name gelatin dessert)
Black Light (uv Light)
Pot (the kind you use to boil water)
step 2Make the Jello
What to do:
Boil 1 cup of Tonic water
Pour boiling tonic water into a jello mould or bowl,
Sprinkle in Jello powder while mixing the hot tonic water, stir until all of the powder is dissolved
Add 2 tbs of sugar to sweeten (if desired)
Add 1 Cup of cold tonic water and mix briefly (1 or two stirs)
Put in fridge for 2 hours
When the jello has set you can take it out and it will glow under a black light.
This is due to the quinine in the tonic water (the stuff that makes it bitter)