Combine all of the ingredients in a bowl. Start with the 2 tablespoons of milk and add more if needed to get the consistency you want. Add food coloring if you would like it colored.
The corn syrup can be left out but it helps make it look glossy and nice.
Notes
The consistency you want might depend on how you are putting it on the cookie. If you use a piping bag or spread it with a knife it can be thicker, but if you dunk it, a bit thinner is nice!