Preheat oven to 350 degrees and spray a petit four baking pan with pam baking spray.
Using a large bowl, whisk until combined the eggs, oil, and milk until combined and smooth.
Divide the batter into four bowls.
Mix in the pink, blue, and purple gel food coloring into their own bowls, leaving the 4th bowl color free.
Spoon 1 tsp of each batter into the petit four cavities.
Bake in the oven for 15 minutes or until a toothpick comes out clean.
Remove from the pan and place onto a wire rack.
Allow the petit four to cool completely.
Using one of the vanilla frosting containers, scoop into a medium bowl.
Microwave for 15 - 20 seconds or until the frosting is liquid-like.
Carefully pour the frosting over the petit fours to completely coat the cakes.
Allow the frosting to firm up.
Using 4 bowls, spoon about 1⁄4 C of the second frosting container into each bowl.
Mix in a few drops of each color into their own bowl and mix until the colors are combined.
Scoop the pink and blue frosting into the piping bag fitted with the star tip.
Scoop the purple and the black frosting into the piping bag fitting with the number 2 tip.
Using the pink and blue frosting, pipe “hair” onto the unicorn.
Using the purple frosting, pipe little dots into the “hair”.
Using the black frosting, pipe details of the eyes on the front of the petit four.
Carefully place the unicorn horn sprinkle onto the top center of the petit four.
Allow the frosting to firm up for 30 minutes before enjoying!