Add the chopped apples to a 6-quart slow cooker.
Pour in the maple syrup, apple cider, and lemon juice. Add the cinnamon, nutmeg, cloves, allspice, and sea salt.
Stir everything together until the apples are evenly coated.
Cover and cook on LOW for 8–10 hours or HIGH for 4–5 hours, until the apples are very soft and beginning to break down.
Use an immersion blender to blend the apples directly in the slow cooker until smooth and creamy.
Stir in the vanilla extract.
Continue cooking on LOW with the lid slightly cracked for 1–2 hours, stirring occasionally, until the apple butter is thick, dark, and spreadable.
Let the apple butter cool completely before transferring it to jars or airtight containers.