Preheat oven to 350°. Line an 8-inch square pan with aluminum foil leaving overhang, and spray with cooking spray, or grease and flour the pan; set aside.
To a large bowl, add the brownie mix, oil, water, and stir to combine; don't over mix.
Turn all but ½ cup batter out into the pan, smoothing the batter with a spatula as needed; reserve remaining batter.
Evenly drizzle the Nutella over brownie batter in pan, lightly smoothing with a spatula as needed. Tip: Make drizzling and spreading the Nutella easier by microwaving it for 5 to 10 seconds.
Evenly sprinkle the chocolate chips, then evenly dollop the remaining ½ cup brownie batter over the chocolate chips, lightly smoothing with a spatula as needed. Note that remaining batter won't completely cover the surface and there will be 'bare patches' with exposed Nutella and chocolate chips.
Bake for about 25 minutes, or until edges have set and center is no longer overly glossy or wet-looking. The toothpick test is very unreliable because you'll likely hit chocolate chips, chocolate chunks, or Nutella, making the toothpick look wet even though brownies are done. I err on the side of under-done than over-done and 25 minutes in my oven allowed for very fudgy, gooey brownies. If you prefer more well done brownies, bake a bit longer. The brownies firm up quite a bit as they cool.
Allow brownies to cool in pan for at least 30 minutes before drizzling with hot fudge, slicing, and serving. Brownies will keep airtight at room temp for up to 1 week or in the freezer for up to 6 months.