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

Add 6 tablespoons of the measured 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. Avoid high speed mixing, as this incorporates too much air.

Adjust the Base: If the mixture appears too thick and paste-like to pour easily, add the remaining 2 tablespoons of milk, one tablespoon at a time, until the icing is smooth and lump-free. The base icing should be somewhat thick but easily stirred.

Achieve Outline Consistency (Piping): Separate about one-third of the icing into a smaller bowl for outlining. Test the consistency by lifting a spoonful and letting it drizzle back into the bowl. The perfect 'outline' line should disappear/melt back into the surface between 8 and 10 seconds. Adjust thickness by adding a teaspoon of powdered sugar (if too thin) or 1/4 teaspoon of water (if too thick).

Achieve Flooding Consistency (Filling): Return to the remaining two-thirds of the icing (the 'flooding' portion). Add milk or water, 1/2 teaspoon at a time, stirring gently. The line should melt back into the surface of the icing in approximately 5 to 7 seconds. This thinner consistency is ideal for filling in the cookie area.

Coloring: Divide the outline icing and the flooding icing into smaller containers based on your desired colors. Use concentrated gel food coloring (avoid liquid food coloring which dilutes the consistency). Add color using a toothpick dipped into the gel until the desired shade is achieved.

Decorate: Transfer the outline icing into piping bags fitted with a small round tip (e.g., PME 1 or Wilton 2). Transfer the flooding icing into piping bottles or larger piping bags. Pipe outlines around the perimeter of the cooled cookies first, letting the outline set for 10-15 minutes to act as a dam.

Flood and Dry: Flood the outlined area with the runny flooding icing, using a toothpick or small scribe tool to gently push the icing to the edges. Allow the decorated cookies to dry completely on a flat surface. This process takes between 8 and 24 hours, depending on humidity and the thickness of the glaze.