Regardless of the number of eggs you add to a brownie mix, they perform an important function. Egg yolks contain fat and protein, while the whites contain mostly water. When added to a brownie mix, eggs emulsify the dry ingredients, creating a moist, light brownie. As the protein in the eggs reacts to heat, it bonds the brownie mixture together to create a firm, semi-solid mass.

If you add an extra egg to brownie mix, you'll get a cake-like brownie, rather than a dense, chewy brownie. The extra egg adds volume and creates a soft, light texture. Most brownie mixes offer directions on how to alter a standard recipe to make cake-like brownies. Add the standard amount of oil and water, but simply add one additional egg. Use a light hand when incorporating all the ingredients to maintain a fluffy texture. You can also add a bit of corn syrup or milk for extra moisture.

Brownie mixes don't always detail the type of eggs to use, but in general, you should use large eggs. If you opt for medium eggs, add one additional egg. If you're using extra large eggs, you can probably reduce the amount of eggs needed by one. Whether you buy conventional, organic, cage-free or omega-3 eggs is mostly a matter of personal preference and budget. You probably won't notice a difference in your brownies. Regardless of the type of eggs you buy, choose fresh ones with no cracks.

To make the most of your baking experiences, try setting eggs and other cold baking ingredients, such as milk and butter, on the countertop 30 minutes to one hour before baking. Allowing eggs and other ingredients to come to room temperature ensures light, fluffy baked goods because the ingredients blend together better. When beaten together at room temperature, these ingredients form pockets of air. In the oven, these pockets of air allow the baked good to rise.