Preheat oven to 350 degrees. Line a muffin pan with 2 liners and spray with nonstick cooking spray.
In a bowl, add egg white and sugar and whisk until combined. Add in vanilla and melted butter and stir until mixed. Add flour, baking powder and salt and stir until smooth. Stir in milk. Divide batter equally between the 2 cupcake liners.
Bake at 350 for 12-15 minutes, or until cake is set. Let cool completely, then frost as desired. I used vanilla frosting mixed with blue and pink food coloring, and piped it on with a Wilton 1M tip.