First, sift the powdered sugar into a mixing bowl. Stir in milk, corn syrup, and vanilla until combined.
Add more milk to thin out the icing, or more powdered sugar to thicken. It should be a pipable consistency.
If you're coloring your icing, divide it into bowls. Add food coloring.
Scoop the colored icing into piping bags.
Pipe the frosting over your cookies any way that you'd like. See the full post for cookie decorating tips!
Allow the icing to harden fully before you serve or store your cookies.