1Heat oven to 350°F (325°F for dark or nonstick pans). Place paper baking cup in each of 24 regular-size muffin cups.
2Make and bake cake mix as directed on box for cupcakes, using water, oil and egg whites and adding lime peel and 1 teaspoon of the vanilla. Cool 10 minutes; remove from pans to cooling racks. Cool completely.
3In large bowl, beat butter and cream cheese with electric mixer on medium speed until well blended. Beat in powdered sugar until smooth. Add lime juice, remaining 1 teaspoon vanilla and the food color; beat until frosting is smooth and spreadable. Frost cupcakes.