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Up the Fat
Maybe you've seen this recommendation on the back of the brownie mix box: Add an additional egg [or just the egg yolk] for a fudgier brownie. In his new book, How to Bake Everything, Mark Bittman concurs: Stir in an egg yolk, along with the rest of the eggs called for in the recipe, and you'll end up with a denser treat. The reason: There's fat in the yolk, which contributes richness; whereas, the whites are lean and produce a more cake-like texture.