In a deep bowl, add the contents of your cake mix, eggs and oil. Mix until everything is just blended. Don't overmix. This batter will be thick.Add white chocolate chips, dark chocolate chips, peanut butter chips and m&m candies. Mix everything with a wooden spoon.Spray a square cake pan (Dimension: 8x8x2) with oil spray. Then, drop your batter into this pan and spread it with your hands until the entire pan is covered.Swirl Nutella into this batter with a butter knife.Bake at 350 degrees for 20 minutes if you like your bars very soft. Or bake them for 25 minutes if you like your bars to be firm.And you are done! Let them cool at room temperature and then use a sharp knife to make rectangular slices. If you have some leftovers, store in the refrigerator.