I've made these brownies several times since I posted the recipe, and I've started to realize that, while the dairy version is indeed the ultimate, the parve version could stand some improvement. The main issue, I think, is that the saturated fat in butter gives the brownies a lush, moist texture that the margarines I've used don't match. Hydrogenated oils should theoretically provide the same texture, but Fleischmann's Unsalted Margarine probably doesn't have enough. (It is significantly softer than butter at the same temperature.) Recently, I tried Smart Balance, and the brownies were downright dry. In this case, I think that the issue was simplify fat: Regular Smart Balance is 67% fat, while butter is about 80%. The package says that Smart Balance is "great for cooking and baking," but don't be fooled.
One option would be to switch to Crisco vegetable shortening, a favorite of a friend and former roommate of mine. Vegetable shortening (like regular margarine) is somewhat out of favor right now because it is high in trans fat, which, though once thought to be more healthful than saturated fat, is now generally believed to be worse. Nonetheless, as I've been implying, it takes a bit of saturated or trans fat to achieve that butterlike texture. Vegetable shortening has less water than most margarine, so it may be just right.
Anyhow, I'll keep you posted. Not right away, though. I really do have to cut back.
(Cross-posted to Kosherblog.)