One of the simplest ways to thicken apple butter is to continue cooking it. Simmer the apple butter over low heat, stirring frequently, until it reduces and thickens to your desired consistency. This can take some time, but it's an effective method. Or mix cornstarch with a small amount of cold water to create a slurry, then add it to the apple butter while it's simmering. This can help thicken the mixture quickly. Keep in mind that cornstarch can affect the flavor and might create a slightly cloudy appearance.