A lot of baking recipes say to add eggs “gradually” or “one at a time” to dough and batter. You add eggs slowly, just as you should with any wet ingredient, to allow them to mix in evenly, says James Peterson, author of Baking, particularly if the mixture contains butter. “If they’re added all at once, they won’t let themselves be worked in,” he says.
Rose Levy Beranbaum, author of Rose’s Heavenly Cakes, explains that “almost all liquid ingredients are added gradually” so that they can emulsify with the fats in the mixture. She says eggs should be added one at a time to cookie dough, bread dough, and layer cake batter.