Place all ingredients in a lightly greased slow cooker and mix until combined.
Cover and cook on low for 4 hours, or until pears are tender.
Transfer 2 cups of mixture to a blender. Remove center piece of blender lid and secure lid on blender. Place a towel over opening of lid and process until smooth, about 30 seconds. Transfer to a bowl. Repeat blending process until all of the mixture is a smooth, butter-like consistency. Return mixture to slow cooker.
Uncovered, cook pear mixture on high heat in slow cooker for 3-4 hours, or until desired thickness achieved, stirring occasionally.
Turn off heat and let pear cranberry butter cool to room temperature. Spoon the mixture into sterile containers. Cover and refrigerate for up to 2 weeks.