Peel and core the apples. Chop into 1" pieces and place in a food processor fitted with the steel blade. Working in batches, pulse the apples until they are in very small pieces and transfer to the slow cooker. If you do not have a food processor, just grate the apples into the slow cooker. (You should be left with about 3½–4 lbs of chopped or grated apples after peeling and coring.)

Add the sugar, cinnamon, nutmeg and salt to the apples in the slow cooker and mix well. Cover and cook on high for 5 hours, stirring once or twice every hour.

After 5 hours on high heat, remove the lid and reduce the heat to low. Cook the apples uncovered for 1–1½ hours, stirring occasionally, until the mixture is thickened and becomes a glossy dark brown color. (Most of the pieces of apple should have cooked down, but if some chunks remain, you may purée the mixture in the blender for a smoother texture.)

Turn the heat off and stir in the vanilla. (At this point, you may also add a bit more sugar or cinnamon to adjust the flavor to your specific taste.)

Transfer the finished apple butter into airtight containers, cover and refrigerate or freeze until ready to use.