Heat oven to 350ºF (325ºF for dark or nonstick pans). Place paper baking cup in each of 24 regular-size muffin cups. Make cake batter as directed on box. Spoon about 1 rounded tablespoonful batter into each muffin cup, using about half of the batter. (Muffin cups will be about one-third full.) Refrigerate remaining batter. Bake 11 to 15 minutes (14 to 20 minutes for dark or nonstick pan) or until toothpick inserted in centre comes out clean. Remove from pan to cooling rack. Repeat with remaining baking cups and batter. Cool cupcakes completely, about 15 minutes.