Combine the sifted confectioners’ sugar, light corn syrup, vanilla extract, and pinch of salt in a large mixing bowl.

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 incorporating too much air can cause bubbles when decorating.

Check the mixture. If it is too thick to pour easily, add the remaining 2 tablespoons of milk, one tablespoon at a time, until the icing is smooth and lump-free. This is your thick base icing.

Separate about one-third of the base icing into a smaller bowl. This portion will be used for outlining and fine detail work.

Test the consistency (the “10-second rule”): Lift a spoonful and let the icing drizzle back into the bowl. If the line disappears completely in 8 to 10 seconds, the icing is at the perfect outline consistency. It should hold a stiff peak when piped.

Adjust if necessary: If the icing is too thin, add one teaspoon of sifted powdered sugar at a time. If 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 until quite runny.

Test the consistency (the “5-second rule”): When you drizzle the icing back into the bowl, the line should melt back into the surface of the icing in approximately 5 to 7 seconds. This allows the icing to smooth out perfectly once applied to the cookie.

Divide the outline icing and the flooding icing into smaller containers based on the colors required. Use concentrated gel food coloring (a tiny dip of a toothpick is usually enough) and stir gently until the desired shade is achieved.

Transfer the outline icing into piping bags fitted with a small round tip (e.g., PME 1) or a fine snipped corner. Transfer the flooding icing into piping bottles or larger piping bags.

Pipe outlines around the perimeter of the cooled cookies first, and allow the border to set for 10-15 minutes. Flood the outlined area with the runny flooding icing, using a toothpick or scribe tool to guide the icing to the edges.

Allow the decorated cookies to dry completely and undisturbed. This process takes 8 to 24 hours depending on the thickness of the icing and ambient humidity. The final result will be a glossy, hard glaze.