1. Microwave butter in a microwave-safe bowl at HIGH 30 seconds to 1 minute or until melted; stir in vanilla and salt.
2. Stir together granulated sugar and brown sugar in a medium bowl.
3. Layer potatoes and sugar mixture in a lightly greased 6-qt. slow cooker, beginning with potatoes and ending with sugar mixture. Pour butter mixture over top. Cover and cook on LOW 4 hours or until potatoes are tender.
4. Transfer potatoes to a serving dish using a slotted spoon, reserving liquid in slow cooker. Keep potatoes warm.
5. Remove 1/3 cup liquid from slow cooker; whisk cornstarch into 1/3 cup liquid until smooth. Carefully pour remaining liquid from slow cooker into a medium saucepan. Whisk in cornstarch mixture. Bring to a boil over medium heat; cook 1 to 2 minutes or until thickened. Pour over potatoes. Serve immediately.
The recipe name "candied yams" is a bit of a misnomer. Orange sweet potatoes are often labeled as yams, but in truth, yams are in an entirely different family of root vegetables. A true yam is starchier than a sweet potato, but also sweet. Yams come from Africa and the Caribbean, and must be cooked in order to be safe to consume. Cooking a yam is laborious: it can be difficult to peel the thick skin, and it must be pounded and boiled.