A rich, chewy brownie might taste like a supremely decadent indulgence, but they're surprisingly adaptable to lower-fat alternatives. Brownies are supposed to be moist, dense and chocolatey, so substitutions are less problematic than they'd be in a delicately flavored and light-textured cake. For example, many fat-conscious bakers use mashed ripe bananas as a replacement for the oil called for in the brownies. The brownies remain moist and soft, and their intense chocolate flavor masks the bananas' distinctive taste.
In Brownie Mixes
If you make your brownies from a mix, the substitution couldn't be easier. Just choose a very ripe banana -- the softer the better -- and mash it to a soft puree. Substitute the banana directly for the oil called for in the mix; if your box calls for 1/2 cup of oil, you'd use 1/2 cup of banana instead. The batter might be slightly drier than usual, so some bakers add an extra tablespoon of liquid. This isn't always necessary, but add it if your brownies come out more cake-like than you want.
Commercial boxed mixes contain a number of stabilizers and emulsifiers that help them adapt to ingredient substitutions. You don't have that luxury with scratch-made brownies, but bananas still work as a substitute for oil. The first time you try the substitution, bake the brownies with no other alterations in your standard recipe. If they come out cake-like, adding slightly more liquid will restore their chewy denseness. If their texture is leathery, or if the edges bake to an unpleasantly hard crust, you'll need to make a few more tweaks.
Fat does two things in your brownie batter. It adds richness and moisture to the crumb, and it softens their texture by weakening the flour's ability to form chewy strands of gluten. Bananas score highly on the richness and moisture side of that equation, but they do little to soften the texture of your brownies. Compensate for that by adding a small amount of buttermilk or lemon juice as part of your liquids, because acidity also weakens gluten. Increasing your recipe's sugar by a tablespoon or two will also soften the crumb, or you can reduce the amount of gluten by using cake or pastry flour instead of all-purpose flour.
Bananas aren't the only suitable substitutes for oil. Other fruit or vegetable purees can work just as well, from applesauce to pumpkin puree. Many bakers use finely textured baby foods, such as carrot puree or prune puree, rather than making their own. The strong chocolate flavor of the brownies masks the original flavor of these add-ins, and in some cases, they bring an earthy or tangy flavor that complements and accentuates the chocolate. A few test batches will quickly tell you which are your favorite alternatives, and disposing of the test batches is seldom an issue.