1Heat oven to 350°F. In large bowl, beat butter and 1 cup powdered sugar with electric mixer on medium speed, or mix with spoon. Stir in flour, cornstarch, lime peel and vanilla until well blended.
2Shape dough into 3/4-inch balls. Place about 2 inches apart on ungreased cookie sheet. Press bottom of glass into dough to grease, then dip into granulated sugar; press on dough balls until 1/4 inch thick.
3Bake 9 to 11 minutes or until edges are light golden brown. Remove from cookie sheet to wire rack. Cool completely, about 30 minutes.
4In small bowl, stir together 1 cup powdered sugar and the lime juice. Stir in additional juice if necessary. Spread glaze over cookies. Squeeze drops of decorating gel on glazed cookies; drag toothpick through gel for marbled design.