There is also commercially made stuff you can get at most grocery stores. Don't know the brands, but I can only tell you that I used one once in a cake and it fell apart, so I'm not sure how good they are.
I find that its best to just use a vegan brownie recipe from scratch. Brownie mixes do not take well to subs at all (especially not Ener-G... I've never made anything so strange in my life!)... I'm sure you can find one in almost any vegan cookbook/bakebook and thousands online. best of luck.
From experience, I'd go with Rabid and Greensgood.... With a typical brownie mix, the Ener-G will make this chocolate humus (rock -- not hummus) with water and oil all around it. I think applesauce would taste the most neutral, though melted chocolate would make me .