A black light cake definitely stands out at a Halloween or rave themed party. While you may have seen black light paints and accessories, finding edible black light reactive items is much more challenging. You cannot purchase black light frosting in the grocery store but you can mix your own at home by simply adding tonic water to any frosting recipe.
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Bring a black light into your kitchen when you are mixing your frosting and decorating the cake to ensure that the cake gives off a strong glow when it is served. If you are working in a sunny kitchen, draw the shades to darken the room as much as possible. You do not have to work in the dark, but turn off the lights every now and then and shine a black light on the cake to monitor your progress.
Tonic water glows a bright blue under a black light because it contains a substance called quinine. The quinine found in tonic water is safe to eat so you can add it to frosting and decorate a black light reactive cake. While tonic water is definitely edible, it does have a bitter flavor. Too much tonic water will create bitter frosting and could ruin the flavor of your cake even if it does look incredible under a black light. You will need to experiment with adding tonic water to frosting to create the desired visual effect without sacrificing flavor.
Mix up black light reactive buttercream frosting by beating together 1/2 cup shortening and one stick softened butter in a mixing bowl. Add 4 cups of confectioners' sugar, 1 tsp. vanilla extract and 2 tbsp. tonic water. Stir everything together until the frosting has a uniform texture and consistency. This will produce frosting that emits a light blue glow under a black light. 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 remember that the frosting will glow blue regardless of what color it is in regular light.
Instead of making homemade frosting for your cake, you can add 2 tbsp. of 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 determine if you are satisfied with the glow. Add an additional tbsp. of tonic water at a time as necessary. If the frosting becomes too thin due to the tonic water, stir in a few tbsp. of confectioners' sugar to thicken it up before you frost the cake.
While you can frost your entire cake with black light frosting, you may only want to add glowing decorations to the cake instead. Scoop your black light frosting into a sandwich sized zippered plastic bag and snip off one corner of the bag. Pipe your black light frosting onto any frosted cake to spell out a message or add a border. This is a great time saver as you can purchase a frosted cake from a grocery store and add black light decorations to it quickly.