1Heat oven to 350°F. Place mini paper baking cup in each of 72 mini muffin cups. Make cake mix as directed on box for cupcakes, using water, oil and eggs. Divide batter evenly among muffin cups. Gently press 1 chocolate chunk into center of each muffin cup.
2Bake 11 to 12 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes; remove from pans to cooling racks. Cool completely, about 30 minutes.
3Frost cupcakes with vanilla frosting. Top each with 1 walnut half. Using red gel, pipe along edges of walnut halves to look like brains. Store cupcakes loosely covered at room temperature.