Core and chop the apples into approximately 1/4-inch pieces. Leave the peels on.
Place the apples in a large slow cooker.
Combine the brown sugar, granulated sugar, cinnamon, and salt.
Pour the sugar mixture over the apples. Add the lemon juice and stir well.
Cover and cook on LOW for approximately 10 hours, until the apples are extremely soft.
Use an immersion blender directly in the slow cooker to purée the apples until completely smooth.
Stir in the vanilla extract.
Leave the slow cooker uncovered and continue cooking on LOW for approximately 2 hours, stirring occasionally.
For thicker apple butter, continue cooking uncovered until it reaches your preferred consistency.
Cool before transferring to airtight containers and refrigerating or freezing.