Prepare Cupcake Batter: Preheat oven to 350°F (175°C). Line a 12-cup cupcake tin with liners. Prepare the white cake mix according to the package directions, adding 1/4 cup of tonic water to the batter.
Bake Cupcakes: Fill the cupcake liners about 2/3 full. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Cool Completely: Let the cupcakes cool completely in the tin before frosting.
Make the Frosting: In a mixing bowl, beat the softened cream cheese and butter until smooth and creamy. Gradually add the powdered sugar, mixing on low speed until combined. Stir in the vanilla extract and 1/4 cup of tonic water. Add edible glow-in-the-dark food gel (if using) to enhance the glow.
Tint Frosting: Divide the frosting into three bowls. Tint one bowl with blue food coloring, one with black food coloring, and leave one bowl white.
Frost Cupcakes: Frost the cupcakes with the tinted frostings, creating a moonlit sky effect. You can use a piping bag and tips for a more elaborate design, or simply spread the frosting with a knife.
Add Details: Use white decorating gel to add details like stars, clouds, or a crescent moon.
Glow Time: To see the glow, turn off the lights and shine a black light on the cupcakes!