Prep the apples: Peel, core and roughly chop the apples.
This is a large batch, so unless you have an extra-large slow cooker, I recommend dividing the recipe between two slow cookers or making two separate batches.
Add the chopped apples to the slow cooker.
Sprinkle the brown sugar, granulated sugar, cinnamon, nutmeg, cloves and salt over the apples. Toss everything together. Cover the slow cooker.
Cook on LOW for 8–10 hours or HIGH for 4–5 hours, until the apples are extremely soft and easily fall apart.
The apples will release quite a bit of liquid as they cook. That’s completely normal.
Use an immersion blender to puree the cooked apples directly in the crockpot until completely smooth. (You can also carefully transfer the apples to a blender and puree them in batches.)
Stir in the lemon juice and vanilla.
At this stage, the mixture may look like applesauce. That’s exactly what you want.
Now we turn it into apple butter.
Reduce until thick: Turn the crockpot to HIGH and leave the lid OFF so the moisture can escape.
Continue cooking for approximately 2–4 hours, stirring every 20–30 minutes.
As the apple butter gets thicker, stir more frequently to prevent the edges from scorching.
The exact amount of time will depend on your apples, the size and shape of your slow cooker and how much moisture they released.
Check the consistency: Your apple butter is ready when it is noticeably darker, glossy, thick and spreadable.
Run a spoon or spatula across the bottom of the crockpot. The line should remain visible for a moment rather than immediately filling back in.
You can also place a small spoonful on a plate. It should hold its shape rather than releasing a ring of watery liquid around it.
Remember that apple butter will continue to thicken as it cools.
Once the apple butter has reduced, taste it.
Apple varieties can vary quite a bit in sweetness. If you’d like your apple butter sweeter, stir in additional brown sugar to taste and let it cook for another 15–20 minutes.
I prefer doing this at the end because the flavor and sweetness become much more concentrated as the apple butter reduces.