You want your apple butter to thicken before you remove it from the heat and allow it to cool for storage or serving. An excellent way to know when it is thick enough is to check if the apple butter fully coats the back of your spoon. Or place a spoonful of apple butter on a plate or cutting board, if it stays mounded keeping it shape without a lot of weeping liquid, it should be ready. Keep in mind that the apple butter will continue to thicken as it cools, producing a sweet and velvety spread. Apple butter will continue to thicken slightly as it cools, producing a velvety sweet spread.