Peel and core the apples. Then, cut them into cubes and place them in the slow cooker.
Mix the sugar, brown sugar, cinnamon, and nutmeg together in a medium mixing bowl. Sprinkle the vanilla extract over the apples, and then sprinkle the sugar mixture all over the apples. Stir until coated.
Cover and cook on LOW for 10 hours. Remove the lid, turn the slow cooker to HIGH, and cook for 1-2 more hours until thickened. (see note)
Use an immersion blender to puree everything into smooth apple butter. (Or use a blender, working in batches, and not filling more than halfway full at a time.)
Pour into hot, clean jars and then close them with a lid. Let cool completely. (see note)
Notes
Cooking time will depend on how juicy your apples are and how thick/reduced you like your apple butter. If you want a very dense consistency, let it reduce even more. Check in with the apple butter every hour. You can also cook it on LOW overnight and then cook it on HIGH afterwards.
If you fill the apple butter into clean glasses when it’s still hot (more than 190°F) and close them right away, they will stay good in the fridge for at least 1 month. For longer keeping, canning will make the apple butter last longer. Be careful to clean the glasses, lids, and rims properly.