Peel and core the apples, cut them into chunks, and transfer them to a 4-quart or larger slow cooker.
In a separate bowl, combine the brown sugar, apple cider, cinnamon, nutmeg, cardamom, allspice, salt, and vanilla extract.
Pour the mixture over the apples, stir to coat, then cover and cook overnight on low heat (10-12 hours).
Once cooked, blend the apple mixture until smooth with a hand-held immersion blender (see notes).
Re-cover and cook for an additional 30 minutes on high heat to finish thickening the mixture.
Allow to cool to room temperature, stir in the lemon juice and spoon into mason jars. Cover tightly and refrigerate for up to 1 week.
Notes
If you don't have an immersion blender, you can use a conventional blender to purée the apple butter, but be sure to work in batches though to avoid over-filling it with the hot mixture.