In a mixing bowl, combine the sifted powdered sugar, milk, corn syrup, and vanilla extract.
Whisk the mixture until it's smooth and well blended. If you desire a thicker consistency, add more powdered sugar; for a thinner consistency, add more milk, a little at a time.
If you want colored icing, divide the mixture into separate bowls and add a few drops of food coloring to each. Stir until the color is evenly distributed.
Transfer the icing into piping bags or squeeze bottles for easy application.
Decorate your cooled sugar cookies as desired, using different colors and piping techniques.
Allow the icing to set for at least 30 minutes before handling the cookies to ensure it hardens properly.
Store any leftover icing in an airtight container for up to one week.