Using an apple corer, remove apple cores from about a fourth of your apples and place in a food processor. Pulse until you have tiny cuts less than ½-¼" chunks. Place in the slow cooker. Repeat with remaining apples.
In a medium bowl, combine brown sugar, sugar, and spices. Pour sugar mixture over the apple pieces in the slow cooker and stir until well combined.
Place lid on slow cooker and cook on LOW heat for 10 hours, stirring occasionally.
Once 10 hours has elapsed, use an immersion blender to puree apples until smooth and no chunks remain. (Or if you don't have an immersion blender, use a standard blender to puree in small batches.)
Add the vanilla and lemon juice (or powder). Continue cooking UNCOVERED for another 2 hours or until the apple butter reaches the desired consistency. Stir occasionally during this process to keep the surface from creating a thick film.
Store in airtight containers and process in a boiling water bath (5 mins for half-pints) or freeze. Once opened, apple butter will keep two weeks in the fridge.