Add sweet potatoes and apples to a slow-cooker.
Add next five ingredients.
Cover and cook on low for 10 to 12 hours.
Remove cover and puree with an immersion blender. You may use a blender or food processor but take care transferring the hot mixture.
Taste for sweetening and season and adjust accordingly.
If mixture is too soupy for your liking, turn the slow-cooker to high, leave uncovered and cook, stirring occasionally, until it reaches the desired consistency.
Add one tablespoon of vanilla extract and stir.
Spoon into jars or storage containers.
Refrigerate or freeze.
Keeps in refrigerator for about 3 weeks.