Heat oven to 350°F (325°F for dark or nonstick pans). Place paper baking cup in each of 58 mini muffin cups.
Make cake batter as directed on box. Divide batter evenly among muffin cups, each about two-thirds full. If necessary, refrigerate any remaining batter until ready to use.
Bake 11 to 14 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes; remove from pan to cooling rack. Cool completely, about 30 minutes.
In small bowl, stir powdered sugar and 3 tablespoons milk until smooth. Add additional milk 1 teaspoon at a time, until desired spreading consistency. Frost cupcakes. Top with rock candy pieces. Store loosely covered.