Combine first 5 ingredients in a 3 1/2- or 4-quart slow cooker. Cover and cook on LOW 2 hours or until melted. Stir chocolate mixture. Add cashews and vanilla, stirring well to coat cashews. Drop nut mixture by heaping tablespoonfuls onto wax paper. Let stand until firm. Store in an airtight container. Freeze It!: Clusters may be frozen up to 1 month.