I  like to use both shorting and butter in my cookie cake. Shortening is made up of 100% hydrogenated vegetable oil and contains no water while butter contains 80% butterfat and 20% water. Shortening traps more air bubbles and produces a soft interior texture along with holding its shape during baking. Technically, yes you can use just one if you have it on hand BUT for best results I do recommend both shortening and butter.