Combine pears and juice in a large saucepan or Dutch oven. Bring to a boil; cover, reduce heat, and simmer 45 minutes or until tender. Place pear mixture in a blender or food processor, and process until smooth.
Combine puréed pear mixture, sugar, cinnamon, and cardamom (if desired) in pan; bring to a boil. Reduce heat; simmer, uncovered, 55 minutes or until mixture is thick, stirring frequently. Stir in vanilla. Cool. Store butter in an airtight container in refrigerator up to 2 months.