Place the sifted confectioners’ sugar into a large mixing bowl. Pour in the light corn syrup and the clear vanilla extract.

Add 6 tablespoons of the milk or water to the sugar mixture. Using a whisk or a rubber spatula, stir the ingredients slowly until they are just combined and smooth. Do not overbeat, as this can incorporate too much air.

Check the initial consistency. If the mixture appears too thick, add the remaining 2 tablespoons of milk, one tablespoon at a time, until the base icing is smooth and lump-free.

Separate about one-third of the base icing into a smaller bowl. This portion will be used for outlining the cookies and holding the shape.

Test the consistency: lift a spoonful and let it drizzle back into the bowl. The line should disappear completely in 8 to 10 seconds. This is the ideal 'outline consistency,' and it should hold a stiff peak when piped.

If the icing is too thin (disappears in under 8 seconds), add one teaspoon of sifted powdered sugar at a time. If the icing is too thick, add water, 1/4 teaspoon at a time.

Return to the remaining two-thirds of the icing (the “flooding” portion). Add milk or water, 1/2 teaspoon at a time, stirring gently.

Test the consistency: The flooding icing should be quite runny. When you lift the spoon and drizzle the icing back into the bowl, the drizzled line should melt back into the surface of the icing in approximately 5 to 7 seconds. This is known as the “5-second rule” consistency.

Divide the outline icing and the flooding icing into smaller containers based on the colors you intend to use. Use concentrated gel food coloring (dip a toothpick into the gel) to achieve the desired shade, mixing thoroughly.

Transfer the outline icing into piping bags fitted with a small round tip (e.g., PME 1 or Wilton 2) or a zip-top bag with the corner snipped. Transfer the flooding icing into piping bottles or larger piping bags.

Pipe outlines around the perimeter of the cooled cookies first. Allow the outline to set for 10-15 minutes before filling.

Flood the outlined area with the runny flooding icing. Use a toothpick or small scribe tool to gently push the icing to the edges to ensure complete coverage.

Allow the decorated cookies to dry completely on a flat surface. This can take anywhere from 8 to 24 hours, depending on the humidity and the thickness of the icing layer.