Eggs are so wonderful and they do so much for cookies in baking. As the egg cooks, it holds your cookies together giving them their structure and shape. That’s why you can have some cookie batters that can be super runny and liquidy but if they contain extra eggs they will just puff up when the cookies bake instead of spreading out. The egg keeps it all held together.
And egg yolks. Oh so delicious egg yolks! These guys are essential in chewy rich cookies. The fat in egg yolks tenderizes the dough by shortening the gluten strands, gives your cookies a richer flavor and will also help make cookies chewier. Because egg yolk is a natural emulsifier it makes your cookie batters smooth with a creamy texture. This is why it’s typically recommended to add the eggs in one at a time. When you are emulsifying the eggs into the butter you want to make sure the eggs are emulsifying with the butter and not just other eggs.
Then there’s egg whites. Egg whites on the other hand can dry cookies out so make sure your cookie recipe has enough sugar to make up for the drying effect of egg whites. Some of my favorite recipes eliminate egg whites entirely resulting in super rich dense and chewy cookies.
You may have heard that room temperature eggs are better for baking. If you need to separate egg yolks from egg whites, cold eggs separate better but in baking warm eggs whip better and incorporate more air into your batter than cold eggs. This can cause major changes when baking cakes that call for a lot of eggs in the recipe but when baking cookies, cold eggs do not typically make a difference.
The ratio of sugar to eggs and flour can dramatically change your end cookie. If the ratio of sugar to flour and eggs is high, the cookie collapses creating a denser, chewier cookie. If the ratio is even higher, the resulting cookies are thin and crisp-like. If there are more flour and eggs, than sugar the cookies are light and cake-like. You can totally play around with this in your cookie baking until you achieve the right ratio for your favorite cookie in taste and texture. For more baking hows and whys check out my free ebook How to Bake Cookies written to help you become the best cookie baker ever!