A Plain Cookie is a food item that can be obtained by purchase from the NPC merchants Silver Platter inside Sugarcane Corner or Free Trade or Sales Pitch in the Crystal Kingdom for five bits. One or more were part of the reward for completing the Good Egg quest. Obtaining a cookie is required as part of the It's All About Me quest.
When used, it replenishes 120 points of your Health.
This table lists the effects given to a consumer: