In a small bowl, whisk together the icing sugar, vanilla, and two tablespoons of water until well combined. The mixture should be a little thicker than honey. If it’s too thin, add a few more tablespoons of icing sugar. If it’s too thick, add more water about 1/2 teaspoon at a time.
Dip your cooled cookies into the icing as is or colour with food colouring and allow the icing to harden for a few hours. If you want a swirly marbled effect, split the icing up and colour each one differently then drizzled together. Dip and twist the cookies into the icing and allow to harden for a few hours.