Add powdered sugar to a medium bowl and whisk to remove any lumps.

Add 2 tablespoons milk, vanilla extract, corn syrup, and salt. Stir gently until combined.

Whisk until smooth and glossy, scraping the sides of the bowl to fully incorporate.

Add more milk a teaspoon at a time until the icing reaches a smooth ribbon consistency.

If the icing becomes too thin, add powdered sugar a tablespoon at a time to thicken.

Divide into bowls and mix in gel food coloring if using.

Use a spoon or piping bag to decorate cookies. Tap cookies lightly to smooth the surface.

Let icing set at room temperature for about 2 hours or until firm before stacking.