A black-light cake is a definite showstopper at a Halloween or birthday party. No, you can't purchase black-light frosting in the grocery store, but you can mix your own at home very easily: just add tonic water to any frosting.
How It Works
Tonic water glows bright blue under a black light because it contains a substance called quinine, which of course is safe to eat. Tonic water does taste a little bitter, though; too much of it could ruin the flavor of the frosting, even if the cake does look incredible under a black light. You'll need to experiment to create the desired visual effect without sacrificing flavor.
Bring a black light into the kitchen when mixing the frosting and decorating the cake to make sure that the cake gives off a strong glow. If you're working in a sunny kitchen, draw the shades to darken the room as much as possible. You don't have to work in the dark, but turn off the lights every now and then and shine a black light on the cake to check your progress.
Homemade Black-Light Buttercream Frosting Recipe
Beat shortening and butter in a mixing bowl.
Add confectioners' sugar, vanilla extract and tonic water. Stir until the frosting has a uniform texture and consistency.
If you want to increase the glow, double the amount of tonic water and add enough additional confectioners' sugar to thicken the frosting to your desired consistency.
Add food coloring to the frosting if you wish, but keep in mind that the frosting will glow blue regardless of what color it is in regular light.
Semi-Homemade Black-Light Frosting
You could also add 2 tablespoons tonic water to a can of store-bought frosting. Stir the tonic water in thoroughly and then shine a black light on the frosting to see if you're satisfied with the glow. Add 1 tablespoon more of tonic water at a time as needed. If the frosting becomes too thin, stir in a few tablespoons of confectioners' sugar to thicken it up before frosting the cake.
You can frost the entire cake with black-light frosting, but you may only want to use it to create glowing decorations instead. Scoop the black-light frosting into a ziplock bag and snip off one bottom corner of the bag. Pipe the black-light frosting onto a frosted cake to spell out a message or add a border.