This is a great question! There are multiple ways to do it. You can crack the egg into a bowl and whisk it together, then measure out half of the mixture. Another, easier way to do it is to use just the egg yolk. Especially in a cookie recipe! Using just the egg yolk is a great option for making a small batch chocolate chip cookies recipe because you don’t use the full egg but you get all of the yolk, making the cookie chewier and richer!