Did you know it's a mistake to fold in the flour completely when making brownies? Doing so develops gluten, and too much gluten will make your dense and chewy brownies too light and airy when baked. Instead, you should fold brownies until there are just streaks of flour remaining so that you can be positive they aren't overstirred.
Hungry for more scientifically sound culinary know-how? You can find it in the new Cook's Illustrated cookbook, "The Science of Good Cooking," by the editors of America's Test Kitchen and Guy Crosby, Ph.D. Here are recipes for chewy brownies and classic brownies from the new book.