I use Betty Crocker Fudge Brownies and really just follow the directions on the box for mixing.
One thing i found though is the brownies turn out a little better if you add one more egg, so 3 total.
I also divide batter in half, and use 2-8x8 pans greased with butter flavor shortening. Because on many different occasions, i found it to cook much more evenly.
Use low heat (gas, propane), or use about 13 to 15 coals for temperature.
This is greatly effected however by BTU's and size of grill!!!! I found in my charcoal grill, i am able to achieve about 25 degrees/coal. So understand, you will/may fail attempting the first few times!
Watch diligently because you really want to move the pan around if you are in a "hot spot" area of your grill.
In about 20 minutes, start doing the toothpick test to see progress, and in about 30-60 minutes (again depending on size and BTU's) you are done!