In large bowl, with mixer on low, beat together all ingredients until well combined. Increase speed to high, and continue beating until stiff peaks form. If not using immediately, press plastic wrap directly onto surface of icing and store in the refrigerator for up to 1 week. (Bring icing to room temperature before using.)
To make sprinkles stick to cookies, pipe icing onto unbaked cookie, top with sprinkles, and bake as directed.