Brown sugar have molasses which helps in providing moisture to cookies, making them gooey.
Overmixing develops the gluten which results in hard and dry cookies.
Chilling helps lock the moisture into the dough in the form of ice crystals.
Egg yolks create a chewy texture in the cookies and make them light.
Baking until fully cooked will make cookies dry so bake it only until the surface is crispy but the center is still gooey.
Melted butter helps lock in the moisture and bind the ingredients together.
A small amount of cornstarch gives cookies a soft, pillowy texture.