Why add one egg at a time while making cake batter?

When we are making a simple butter cake or rather basic cake all what you tend to keep ready is the greasing of the cake moulds with butter or oil and setting it aside. The oven is preheating and you've nearly finished creaming the butter and sugar. It's almost time to add eggs to the batter, and the recipe asks you to add one egg at a time with ample mixing between additions. You're wondering, “Why all this fuss when adding eggs to cake batter? Why can't you just pull eggs out of the fridge, crack them, and throw them into the cake mixture?”

All of those extra little precautions like creaming butter and sugar completely, allowing eggs to come to room temperature before adding them slowly, mixing vigorously and thoroughly between each addition, obsessively scraping the sides and bottom of the bowl, plus the beater, are to create a stable emulsion of liquid and fat in the batter. When this is done correctly, the cake will have a springy, even crumb, great flavor and light texture. If the batter is not properly emulsified, the resulting cake can be uneven and flat, flavorless and have a heavy texture. It's even possible for the cake to "fall" or sink into itself when baking. That's the reason why we slowly add eggs to the butter mixture, one at a time.

An emulsion is the suspension of small globules of one liquid in a second liquid with which the first will not mix, such as oil and vinegar. To create an emulsion, we slowly add eggs to the butter mixture, one at a time, beating rapidly to suspend the water from the butter and egg whites in the fats from the butter and egg yolks. Lecithin, an emulsifier found in eggs, helps to stabilize the emulsion, and does aeration from steady, rapid beating.