Peel, core, and slice the apples.
Add the sliced apples to the slow cooker.
Cover and cook on low for 4 to 6 hours, or until the apples are soft and beginning to break down.
Blend the softened apples with an immersion blender until smooth.
In a small bowl, combine the brown sugar, cinnamon, cloves, nutmeg, vanilla, and salt.
Stir the sugar and spice mixture into the blended apples.
Continue cooking on low for 4 to 6 more hours, stirring occasionally, until the apple butter is dark, thick, and spreadable.
If the apple butter seems too thin, set the slow cooker lid slightly ajar so extra moisture can cook off.
Test the apple butter by placing a spoonful on a plate. If it holds its shape and does not run, it is ready.
Stir in lemon juice if using.
Spoon the apple butter into clean jars.
Cool, then store in the refrigerator or freezer.