Ask a roomful of people to describe the perfect brownie, and chances are you'll year some strong opinions on the subject. While no two brownie recipes are exactly the same, they do tend to fall into two broad categories: chewy and cakey. Chewy brownies can range from soft to dense and fudgey. Cakey brownies are lighter and are often served with frosting. Understanding the food science behind these two styles will help you design your own version of the perfect brownie.
The dense structure of chewy brownies requires a strong base to support all that chocolate flavor. Using 1 3/4 cups of all-purpose flour provides enough gluten to give a chewy, elastic texture.
Cakey brownies are built around a light, airy structure. They need significantly less all-purpose flour -- just 2/3 cup -- to provide a foundation for the rest of the ingredients.
Both styles benefit from common all-purpose flour. Cake flour is lower in protein than all-purpose flour, and leaves brownies of both styles with a crumbly texture that does not hold together well.
Eggs provide protein, which strengthens the gluten in the flour. They also act as emulsifiers, helping bind the fat with the other ingredients.
Chewy brownies benefit from an extra egg yolk per whole egg called for in the recipe. The extra protein in the additional egg yolk helps give the brownie its density.
Cakey brownies leave out the extra egg yolk. By using whole eggs only, they maintain a protein level that is similar to that in cake. This results in a cakelike texture.
The fat in brownies gives them the richness you expect from a chocolate treat. The key to chewy brownies is the ratio of saturated to unsaturated fat. Using a mixture of melted butter and vegetable oil will give you a chewier texture than either fat alone. The extra egg yolk also adds fat.
Cakey brownies have less than one-half the fat of chewy brownies, which keeps their texture light and airy.
A brownie's entire reason for being is to showcase chocolate. This is one area where you can experiment freely. Most recipes for either style of brownie call for 1/4 to 1/3 cup cocoa powder.
If your tastes trend toward the chewy side of the brownie world, consider adding finely chopped chocolate chunks as well. Adding bittersweet, semi-sweet or even milk chocolate will give your customized brownies a twist.
If you prefer cakey brownies, you can ramp up the chocolate flavor by coating your baked and cooled brownies in chocolate ganache or buttercream.