Combine all ingredients in a 5-quart electric slow cooker. Cover and cook on low 10 hours or until apples are very tender.
Place a large fine-mesh sieve over a bowl; spoon one-third of apple mixture into sieve. Press mixture through sieve using the back of a spoon or ladle. Discard pulp. Repeat procedure with remaining apple mixture. Return apple mixture to slow cooker. Cook, uncovered, on high 1 1/2 hours or until mixture is thick, stirring occasionally. Spoon into a bowl; cover and chill up to a week.
Stovetop variation: Combine all ingredients in a Dutch oven. Cover and cook over medium-low heat 1 hour or until apples are very tender, stirring occasionally. Strain through a sieve as recipe instructs in Step 2. Return mixture to pan. Cook, uncovered, over medium-low heat 15 minutes or until thick, stirring frequently.
A mixture of apple varieties, rather than just one type, will produce apple butter with rich, complex flavor in this slow cooker recipe. Good choices include Esopus Spitzenburg, Granny Smith, Jonathan, Northern Spy, Rome, Stayman, Winesap, and York. Enjoy the apple butter over toast or English muffins, or serve it with pork chops or chicken.