In a medium mixing bowl, combine the sifted powdered sugar with milk (or water) and light corn syrup.
If using, add vanilla extract to the mixture.
Using a whisk or electric mixer, blend the ingredients together until smooth. Continue mixing until the icing reaches a thick but pourable consistency.
If the icing is too thick, add more milk, a tablespoon at a time, until the desired consistency is achieved.
To color the icing, divide it into smaller bowls and add food coloring as desired, mixing until the desired color is achieved.
Once the icing is prepared, either spread it on the cooled sugar cookies using a spoon or knife, or transfer it to piping bags for more detailed decorating.
Allow the icing to dry completely before stacking the cookies or covering them, which usually takes about 1-2 hours.
For a shinier finish, you can use slightly more corn syrup or adjust the ratios as desired.
Experiment with different flavors of extracts for a unique taste, if desired.