Add the butter to the bowl of a stand kitchen mixer and beat for 5 minutes until doubled in volume.
Gradually add the sugar 1/2 cup at a time and beating between each addition.
Gradually add vanilla and milk. Beat on high 3 minutes, until combined.
Divide buttercream evenly into 3 bowls. Add different food coloring (pink, blue and yellow) to each mixing bowl. With a large serving spoon, add buttercream into a large piping bag. First spoon pink, blue, and yellow, continue pattern till the piping bag is full.
Frost cupcakes with unicorn rainbow buttercream icing using a jumbo size closed star piping tip.