Why baking recipes advise adding eggs 1 at a time, instead of all at once

We've all done it, added all the eggs at once. But does it really matter as long as they're all added? Yes, it does, and here's why.

The same principle applies here: If you add all your eggs slowly, it will give everything a chance to get to know each other, mesh together and become one. If you add the eggs all at once, the butter-fat mixture won’t be able to absorb it all and won’t create a nice suspension. It’s about creating an emulsion The reason why lies on a microscopic level. It first starts with creaming butter and sugar. If you’ve done it correctly, you’ll have a light and fluffy mixture that is pretty much all fat. Now we know fat (or oil) and water don’t mix, at least not without some effort. However, when we add the eggs to our creamed mixture, that’s exactly what we’re attempting to do. We’re trying to suspend the water in the egg into the fat, scientifically know as making an emulsion.