3 pounds crisp, sweet apples, such as Fuji, Gala, Honeycrisp, or Pink Lady (about 7 medium apples)
3 tablespoons pure maple syrup
1 1/2 teaspoons ground cinnamon
1/4 teaspoon ground nutmeg
1/4 teaspoon ground allspice
1 teaspoon pure vanilla extract
Tiny pinch kosher salt
Peel and core the apples and cut them into a rough 3/4- to 1-inch dice. Place in the bottom of a 5-quart or larger slow cooker, then top with maple syrup, cinnamon, nutmeg, and allspice. Stir to combine. Cover and cook on high for 3 to 4 hours, or on low for 8 to 10 hours, stirring once halfway through, until the apples are very tender and brown.
With an immersion blender, puree the apples until the apple butter reaches your desired consistency (I like mine fairly smooth). If you do not have an immersion blender, transfer the apples to a blender or food processor in batches and blend until smooth, then return to the slow cooker.
Turn the slow cooker to high and let the apple butter continue to cook, uncovered, for 2 additional hours or until thickened, stirring occasionally. Stir in the vanilla extract and salt. Taste and add additional maple syrup as desired. Spoon into jars or another airtight container.