Wash, peel, core, and chop the apples into small chunks. (Peeling is optional if you plan to blend the apple butter thoroughly later.)
Place the apple chunks in the crock pot. Add vanilla extract, cinnamon, nutmeg, allspice, cloves, ginger, and salt (if using). Stir well to coat the apples evenly with the spices.
Cover the crock pot and cook on low for 10-12 hours, stirring occasionally, until the apples are very soft and dark brown. (For quicker cooking, you can set it to high for 4-6 hours, but low heat develops the flavors better.)
Use an immersion blender to puree the apples into a smooth, buttery texture directly in the crock pot. Alternatively, transfer the mixture to a blender, blend until smooth, and return it to the crock pot.
If the apple butter is too thin, leave the lid slightly ajar and continue cooking on low for another 1-2 hours, stirring occasionally, until it reaches your desired consistency.
Let the apple butter cool completely before transferring it to jars or airtight containers. Store in the refrigerator for up to 2 weeks, or freeze for up to 6 months.