Sift 1 cup icing sugar into a bowl
Add 1 to 2 tablespoons milk or water
Stir until smooth
Add more liquid a few drops at a time if needed
Add a few drops of vanilla extract if desired
Mix until the icing is thick but spreadable
Add more icing sugar if the icing is too thin
Add food coloring if desired
Spoon or pipe onto cooled cookies
Let the icing set before serving