Prep the apples. Peel, core, and quarter the apples. Smaller pieces break down faster and give a smoother butter in the end. A mix of sweet and tart apples gives the richest flavor; soft varieties like Golden Delicious, McIntosh, or Cortland cook down quickest.
Load the slow cooker. Spray the inside of a 6-quart slow cooker with cooking spray. Fill it with the apples, then add the sugars, spices, and water. Stir well so every piece is coated.
Cook low and slow. Cover and cook on LOW for 8 to 10 hours or on HIGH for 4 to 6 hours, stirring a couple of times if you can. The apples should soften completely and turn a deep golden color.
Thicken uncovered. Take the lid off, give it a good stir, and keep cooking uncovered for 2 more hours on LOW (or 1 hour on HIGH) so the extra moisture can escape and the butter turns thick and spreadable.
Puree until smooth. Blend right in the cooker with an immersion blender until silky. No immersion blender? Ladle the butter into a regular blender in batches and puree, then return it to the cooker.
Jar it. Let the butter cool slightly, then spoon it into clean jars. Refrigerate for 7 to 10 days, freeze for up to 3 months, or can it for the pantry: ladle the hot butter into hot, clean half-pint jars, leaving 1/2 inch of headspace, wipe the rims, seal, and process in a boiling water bath for 10 minutes. Let the jars rest in the hot water 5 minutes more, then set them on a towel to cool completely. Check the seals — lids should stay down when pressed — and store sealed jars in a cool, dark place for up to 1 year.
