With a hand mixer or stand mixer fitted with the paddle attachment, cream the butter on high until creamy and light (almost white) in color, about 7 minutes.
Turn the mixer to low speed and add the powdered sugar one cup at a time, scraping down the bowl and paddle after each addition and making sure each cup is fully incorporated before adding the next one.
Add the vanilla, milk, and salt and mix on low for another minute until fully incorporated and smooth. Fold in the rainbow sprinkles with a rubber spatula.
