1. Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with baking spray with flour.
2. Make cake batter as directed on box, adding peppermint extract with the water. Measure 1 cup batter into small bowl; stir in red food color; set aside. Pour remaining batter in pan.
3. Drop red batter by 12 generous tablespoonfuls randomly onto batter in pan. For marbled design, pull knife through batter and filling in wide curves; turn pan, and repeat.
4. Bake as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
5. Carefully spread chocolate spread evenly over cake. Spread whipped topping over chocolate.
6. Garnish with candies; sprinkle with chocolate. Store covered in refrigerator.