If you want to make the cookies eggless, simply skip the egg and double the milk in the recipe. You're aiming for a cookie dough that's not at all dry but is not too wet either. It should be slightly sticky before going in the refrigerator. Remember though, if you wanna err on either sides then wet cookie dough is the way to go. Wet sticky dough will make gooey cookies. Dry cookie dough will make hard cookies.