Most recipes call for adding eggs one at a time to creamed butter and sugar. Would adding the eggs all at once really make a difference?
Looking into our archives, we noticed that almost all of our cake and cookie recipes that call for more than one egg either add them one at a time or premix the eggs and then add them in a steady stream. We wondered why this extra effort is involved and whether it is necessary. First, we made a batch of our Chocolate Chip Oatmeal Cookies (January/February 1997). When the two eggs in the recipe were added one at a time, it took about 30 seconds to incorporate each into the creamed butter and sugar, compared with slightly over two minutes when both were added at once. While the difference in time might not seem significant, the difference in the finished cookies was. Eggs added one at a time led to cookies that were thick and chewy; eggs added all at once produced cookies that spread, became unevenly shaped, and were not as chewy. We encountered similar differences with our Chocolate Sour Cream Bundt Cake (January/February 2004) and Classic Pound Cake (January/February 2007). In both recipes, when the eggs were added together, it took longer to incorporate them and the cakes turned out denser and slightly rubbery.
The fact is, like oil and vinegar, eggs and butter don’t mix naturally. It’s a matter of chemistry: Butter is at least 80 percent fat, while eggs contain large amounts of water. So any time you add more than a single egg to creamed butter, it’s best to do it slowly to give the mixture time to thicken and emulsify.