Prepare the Cupcakes:
Preheat your oven to 350°F (175°C). Line a muffin tin with cupcake liners. In a medium bowl, whisk together flour, baking powder, and salt. In a separate bowl, cream together butter and sugar until light and fluffy. Beat in the eggs one at a time, then add vanilla extract. Gradually add the dry ingredients, alternating with the milk, and mix until just combined. Stir in tonic water.

Bake the Cupcakes:
Spoon the batter into the cupcake liners, filling each about 2/3 full. Bake for 18-20 minutes, or until a toothpick comes out clean. Let the cupcakes cool completely on a wire rack.

Make the Frosting:
Beat softened butter until smooth. Gradually add powdered sugar, milk, and vanilla, and beat until fluffy. Stir in tonic water until the frosting reaches the desired consistency and glow effect. Add food coloring if desired for extra color.

Frost the Cupcakes:
Once the cupcakes are completely cool, frost them with the glow-in-the-dark frosting. The tonic water will make the frosting glow under UV light (blacklight).

Serve and Enjoy:
Serve the cupcakes under a blacklight for a glowing effect at your party!