Peel and core the apples and cut them into a rough 3/4- to 1-inch dice.
Place the apples in the bottom of a 5-quart or larger slow cooker.
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. Enjoy on toast, on waffles, mixed with yogurt, drizzled over ice cream, on a grilled cheese (YUM), on cornbread, or served with pork.
TO STORE: This apple butter will tastes even better the next day once it has rested in the refrigerator and the flavors are blended. Store leftover apple butter in an airtight container in the refrigerator for up to 2 weeks or freeze for several months. Let thaw overnight in the refrigerator.
*The amount of maple syrup you need will vary based on the sweetness of your apples and personal preference. Taste and adjust as needed. If you prefer to make a no-sugar apple butter, you can simply omit it.