For the smoothest icing, use powdered sugar made with tapioca starch instead of cornstarch to avoid a chalky taste. Always add liquid in very small increments, as a few drops can change the consistency from thick to thin. Stick to gel food coloring to maintain the icing's texture. To prevent a crust from forming while you work, cover the bowls of icing with a damp towel. Allow decorated cookies to dry completely for 6-24 hours before stacking to ensure the designs are smudge-proof.