You'll want to start by creating the buttercream icing. Put the following ingredients together in a large bowl and beat with an electric mixer:  butter, confectioners’ sugar, and milk. Have the mixer on low to begin and then slowly increase to high.   

Once all the ingredients are combined well and are forming peaks that are stiff as you're mixing, it means that the buttercream is done and ready. 

Take about half of the newly mixed buttercream icing and place it into a piping bag. You'll also want to have a round tip on the end as well. Set aside.  

Color the remaining buttercream icing with the blue gel food coloring.

Using a small knife or spatula, frost the cupcakes with the blue frosting.

Then, it's time to add your rainbow. Don't forget to make it in an arched shape as you go, but start by adding the red sprinkles, followed by the yellow, green, blue and purple. 

Pick up your piping bag and squeeze out the "clouds" at the end of both sides of the rainbow.