The ASL sign for cook looks like you are flipping pancakes on a skillet. Your non-dominant hand is flat in front of your body, representing the skillet. Then hold out your dominant hand flat on top of your non-dominant hand, flip it over to reverse sides, and then flip it back.
Cook is a good sign to compound with food items for more advanced babies. For example, when you are frying or scrambling your baby's eggs, you can sign cook and egg.
Toddlers always want to imitate. Teach the sign for cook as soon as your little one begins to show interest in activities related to food preparation.