Reserve 24 candies for later use. Place remaining candies in large freezer-weight resealable plastic bag; freeze 1 hour.
Meanwhile, grate enough peel, then squeeze juice from 2 limes to measure 2 Tbsp. EACH zest and juice. Cut remaining lime into 6 slices, then cut each slice into quarters. Reserve for later use.
Heat oven to 350°F.
Crush frozen candies. Prepare cake batter as directed on package; stir in 3/4 cup crushed candies, and 1 Tbsp. EACH lime zest and juice. Spoon into 24 paper-lined muffin pan caps. Return remaining crushed candies to freezer.
Bake cupcakes 18 to 20 min. or until toothpick inserted in centers comes out clean. Cool cupcakes in pans 10 min. Remove from pans to wire racks; cool completely.
Beat butter in large bowl with mixer until creamy. Gradually add sugar, mixing well after each addition. Blend in milk, 1 Tbsp. at a time, until frosting is of desired spreading consistency. Beat in vanilla, then remaining lime zest and lime juice until blended.
Spread or pipe frosting onto cupcakes; sprinkle with remaining frozen crushed candies. Top with remaining whole candies and lime slices just before serving.