In these cases, we're generally talking about whole eggs. To get an idea for what happens when yolks are used instead of a whole egg, we turned to our new favorite baking resource BakeWise by Shirley O. Corriher.
She advises us to think of an egg as three ingredients that each do something different in the cookie. Egg whites are mostly protein, so they help set the structure and dry baked goods out. Yolks are primarily fats, vitamins, and minerals with some protein, and they make baked goods moist and give a smooth texture. Whole eggs do a bit of both.
Based on this understanding of what the different parts do, our guess is that substituting a whole egg with two yolks would give us a very tender and moist cookie with a cake-like structure. Because we lose the proteins in the whites, the cookie would likely be crumbly. The flavor wouldn't be affected very much, though the cookie would taste richer.
Does anyone have experience with substituting eggs in cookies or other baked goods? What do you think would happen?
Related: Help! How do I Fix My Muffin Recipe?
(Image: Emma Christensen for the Kitchn)