Stir apples, sugar, apple cider, cinnamon, allspice, salt, and cloves together in a slow cooker.
Cook on High for 1 hour. Reduce heat to Low and continue cooking, stirring occasionally, until the mixture is thickened and dark brown, 9 to 11 hours. Remove cover and continue cooking on Low, whisking occasionally, until desired smoothness is achieved, about 1 hour.
Sterilize the jars and lids in boiling water for at least 5 minutes. Pack the apple butter into the hot, sterilized jars, filling the jars to within 1/4 inch of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars into the boiling water using a holder. Leave a 2-inch space between the jars. Pour in more boiling water if necessary to bring the water level to at least 1 inch above the tops of the jars. Bring the water to a rolling boil, cover the pot, and process for 10 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area.