Preheat oven to 350 degrees F. Line an 8 inch square metal pan with foil and spray bottom only with cooking spray.
Pour the cake mix into a mixing bowl and mix out any big lumps. Make a well in the center and add oil. Mix some of the cake mix in with the oil and then add the eggs and vanilla. Continue mixing, adding in the chocolate chips.
Once you have a thick batter, spread it evenly in the pan. Sprinkle more chocolate chips (and nuts, if using) over the top.
Bake at 350 for 25 minutes. I tested mine at 25 minutes and the temperature topped out at 175 F. They looked done, but were very fudgy in the center, so I just let them cool completely and they were fine. The internal temperature of brownies really seems to depend on the recipe, but in this case they were done before the internal temperature reached 200.
Let cool completely. If you want to speed things up a little, you can let them cool at room temperature for about 30 minutes, then lift them from the pan and let them finish cooling in the freezer.