Place the apple chunks in a slow cooker. Stir in the brown sugar, white sugar, and salt. Place the cinnamon stick and sage in the slow cooker. Cover, and set to low for eight hours.

After about eight hours, check the apples. They should be broken down and releasing juices. Remove the sage leaves and cinnamon stick. Discard the sage and set the cinnamon stick aside. Using an immersion blender (or regular blender*), puree the apple butter until very smooth.

Add the vanilla extract and cinnamon stick to the apple butter and stir well. Cover and cook for another two hours on high, stirring occasionally, until dark brown and thickened. You can cook it longer for thicker apple butter. Remove the cinnamon stick and discard. Cool the sage apple butter and store in jars in the fridge for up to two weeks.