How to Make It
Preheat a slow cooker on the low setting for at least 20 minutes.
Meanwhile, peel and core the apples, then slice them thinly. In a large bowl, toss the apple slices with the brown sugar; set aside.
Add the butter to the preheated slow cooker. Once it has melted, stir in the cardamom and whiskey.
Add the apples to the slow cooker, stirring to combine everything. Cover with the lid and cook on the low setting for 18 to 24 hours, until a thick spread forms.
Transfer the apples to a blender or food processor, using a slotted spoon to drain them of any liquid, and puree until smooth. Let the puree cool, and then store it in clean jars in the refrigerator.
The apple butter will keep in the fridge for a couple of weeks, or you could fully process it in sterilized jars according to the jar manufacturer’s instructions and have it be shelf-stable for up to a year.