Prepare the Apples: Peel, core, and slice the apples into uniform chunks.
Combine Ingredients: In your slow cooker or pot, mix the apples, sugars, spices, and salt.
Slow Cook: Set the slow cooker on low and cook for 8-10 hours, stirring occasionally, until apples are deeply softened and fragrant.
Blend Smooth: Using an immersion blender, puree the apples until the mixture is silky smooth. Alternatively, carefully transfer the apples in batches to a blender.
Reduce Further: With the lid slightly ajar, continue cooking for 1-2 more hours to thicken the apple butter, stirring occasionally to prevent sticking.
Add Vanilla: Stir in the vanilla extract during the last 30 minutes of cooking.
Cool and Store: Allow to cool slightly before transferring to sterilized jars. Seal tightly and refrigerate or can according to your preference.