Wash, peel, core and chop apples.
Coat inside of slow cooker with butter and place apples inside.
Sprinkle brown sugar, granulated sugar, cinnamon, nutmeg and salt over apples. Gently stir to mix. Pour water over mixture.
Place lid on slow cooker, set to low for 8 hours.
After cooking time, remove lid and let cool for 30 minutes. Using an immersion blender (regular blender on pulse or potato masher will also work) blend apples until smooth.
With lid off of crock pot, continue to cook apple butter for 1 to 1 1/2 hours until thickened.
Let cool & store in airtight container in refrigerator.