I have a few variations on brownie recipes; all of them are better than any that i've tasted elsewhere.
And they're all simple. think of them as cake. the things that make a light/fluffy cake are all things you want to avoid.
Here's what they have in common: very little flower, very little added sugar, lots of butter, lots of eggs, baked only as long as necesary to set. the richest and thickest contains about 1/3 cup sour cream per 8" square pan. Substituting creme fraiche makes them more delicate but not as gooey.
If you're serious, only use excellent chocolate--60 to 75% cocoa, very high quality chocolate whose melting characteristics you're familiar with. I also avoid recipes with things like canned syrup, corn syrup, marshmallows, etc... for me there's nothing like simple, excellent ingredients with nothing between you and them.