In a medium bowl, combine all the ingredients with an electric hand mixer. Add a bit more milk, if needed, until it’s the desired consistency - it should be thick, but spreadable.
If coloring the frosting with different colors, divide into bowls and color as desired at this point.
Spread on top of cookies with a dull knife or transfer to a piping bag.
Top with sprinkles , if desired (they need to be added right away before the frosting sets.)
NOTE: This frosting sets fairly quickly. It won't become 100% hardened like royal icing, but it does form a "crust" so you can stack your cookies.