There are many things I've learnt about brownies. This has taken many years of extensive testing, all for your benefit. I will never be skinny. See the dedication??
Chocolate v cocoa
Ok, chocolate is expensive! On the other hand, it makes waaay better brownies. It's the combination of chocolate and lack of bicarb that makes brownies brownies and not cake. If you do have a recipe calling for cocoa, just be prepared that it's gonna be lighter and dryer... if you're into that kind of thing.
When buying chocolate, don't bother buying the fancy stuff, or even cooking chocolate (unless you have a large disposable income). I buy the massive slabs of Old Gold, those babies will last you 2-3 batches. Whatever you end up getting, just make sure it has about 70% cocoa.
Sugar is great isn't it? Well, no. Not really. The main change I make to my recipes is to reduce the amount of sugar, there's already almost enough in the chocolate. Generally in my recipes, if I've put 3/4, it was probably a cup to begin with etc. etc.
As a rule, any nuts are better than no nuts (isn't that the rule for pretty much everything?). My favourites are walnuts because they're soft and lower in fat, although that's a minor concern. I also use almonds a lot, pecans when I must and macadamias if I'm using white chocolate/suddenly get a sugar daddy.
The last awesome little tip is from Jamie's 30 minute meals. Baking paper is a must when baking brownies, given the vast amount of sugar. The best way to fit it into the tin is to crumple a good-sized sheet and wet it completely, then squeeze excess water out. This makes it really easy to handle & easy to get into the corners of the pan. Neat huh?