Peel, core and cut apples into chunks. In a large Dutch oven set over medium heat, add apples, brown sugar, butter, cinnamon and salt. Simmer, stirring occasionally, 25 to 30 min., until apples are tender.
Using an immersion blender, puree apples until smooth (or puree mixture in food processor).
Transfer mixture to slow cooker. Cook 4 to 6 hr. on HIGH, stirring every hr., or until mixture is deep amber in colour. Cool completely.
Transfer to airtight containers. Store refrigerated up to 5 days (or freeze up to 1 month). To preserve apple butter in canning jars, follow manufacturer’s instructions.