Peel, core, and quarter the apples.
Spray the inside of a 6-quart slow cooker with cooking spray and place the apples inside. I use as many apples as it takes to fill up my slow cooker!
Add remaining ingredients to slow cooker and stir to combine.
Cover and cook on LOW for 8-10 hours or on HIGH for 4-6 hours.
Uncover, give it a good stir, and continue cooking uncovered for an additional 2 hours on LOW or 1 hour on HIGH. This will allow the apple butter to cook down and thicken.
Use a hand blender to puree until smooth. If you don’t have a hand blender, transfer to a regular blender and puree in batches.
Store in mason jars in the refrigerator for a few weeks, or keep in the freezer. If you want to store the apple butter in a pantry, be sure to follow proper canning procedures (sterilizing and processing jars). See the canning instructions in the notes below.