Pandan Essence is an extract made from leaves of the Pandan (screwpine) plant. The essence is used in South East Asian cooking, primarily for desserts. More often found is a chemically derived artificial flavoring which remains the go-to product in many countries that uses this flavoring. See recipe list below.
Look for the essence in Asian grocery stores or you can purchase it online from several sources including Amazon.com: McCORMICK Pandan Flavor Extract.
If you don't have (or can't find) pandan essence you can: