Dice your apples into ½ inch pieces. Peeling them is optional—if you like a more rustic texture, feel free to leave the skins on. If you prefer a smoother apple butter, peel them first.
In a medium bowl, whisk together the brown sugar, sugar, cinnamon, salt, cloves, and vanilla. This combination of spices is what gives your apple butter its signature fall flavor.
Place the diced apples into the bottom of your slow cooker, then pour the sugar and spice mixture over them. Toss everything to combine, making sure the apples are evenly coated. Cover with the lid and cook on low for 10-12 hours, stirring occasionally. By the end of the cooking time, your apples should be incredibly soft and tender.
Once the apples are tender and have released their juices, transfer everything to a blender. Blend until smooth, working in batches if necessary. If you want a super silky apple butter, you can even pass it through a sieve for an extra smooth texture.
Measure 3/4 cup of this apple butter to use for the apple blondies. Place the excess into a storage container and refrigerate for up to 2 weeks.