Thoroughly wash apples. If they seem waxy, scrub with a produce brush. Organic apples are recommended, since you will be cooking them with the skins on. Quarter the apples and add them to your slow cooker. Add apple cider vinegar and cider or juice. Cook, covered, until the apples are soft.
Mash the apples with a potato masher until thoroughly pulped. Run the pulp through a food mill to catch the skins and seeds, passing a puree. If you do not have a food mill, press the pulp through a sieve.
Add dark brown sugar, maple syrup, ground cinnamon, nutmeg, allspice and salt. Cook, stirring occasionally to prevent scorching, until the apple butter darkens and it mounds on a spoon. Adjust sweetness and seasonings to taste. Store in the refrigerator for a month, or water bath can for 15 minutes and store in a cool, dark place for up to a year.