Heat oven to 350°F (or 325°F for dark or nonstick pans). Place paper baking cup in each of 18 large (2 3/4x1 1/4-inch) muffin cups. Make cake mix as directed on box, using water, butter and eggs. Place 1/4 cup batter in each muffin cup. Spoon 1 tablespoon cold chocolate mixture on top of batter in center of each cup.