Prepare the border icing by mixing the powdered sugar, vanilla extract, milk, and food coloring (if desired) in a small bowl with a spoon. The border icing should be thick and just barely pourable. If it becomes too thick, you can add another ½ tablespoon of milk to thin the mixture. Transfer the icing to a small plastic squeeze bottle and set aside.
Prepare the flood icing by mixing the powdered sugar, vanilla extract, milk, and food coloring (if desired) in a small bowl with a spoon. The flood icing should be slightly thinner and easily pourable. If it becomes too thick, you can add another ½ tablespoon of milk to thin the mixture. Transfer the icing to a small plastic squeeze bottle and set aside.
To decorate your cookies, start with the border icing first. Squeeze the bottle with even pressure and use the icing to draw an outline around the edge of the cookies. You can also add designs inside the border, if desired. Allow the border icing to dry slightly, then switch to the flood icing. Fill the inside of the border lines you created with the flood icing. Once you are finished decorating, allow the icing to set up for 24 hours before storing in an airtight container for up to 1 week.
Recipe by Love Grows Wild at https://lovegrowswild.com/2014/11/2-step-iced-sugar-cookies/