Prop open the lid: The pear butter will thicken better if you prop open the lid.  I set mine across the top, so that either end is open, but the pears are still mostly covered.  This will help the extra moisture from the pear sauce evaporate during the cooking.

Faster still: If you don’t have time to make a batch of pear sauce, peel and seed ripe pears.  Blend them in a blender and use them in place of the pear sauce.  It will take longer for the jam to cook down, but you’ll still get amazing pear butter.  Cook on low for 6 hours, then see what your consistency looks like after the 8 hours on warm.