Cut the pears into quarters; do not worry about the stems or seeds yet. Combine them with everything EXCEPT THE BUTTER in a Ziploc Gallon Bag. Squeeze out the excess air. Does not have to be perfect. Process via Sous Vide @
Shock to 70F/21C, refrigerate to 40F/4C.
Remove mixture from bag and run through a food mill, or blend it, or process it, just mash it all up, and then squeeze it through a strainer. That will catch the seeds. Spread the mixture out on a silicone mat or something that will neither stick to it or scorch it. Those silicone mats are great, and they are cheap now too. They save a lot of clean up.
Bake overnight in your oven @170F/77C, 12 hours usually does it. It will be quite dried out on the edges and darkening a bit. That is good. Stop and smell that. I mean, really STOP and smell the pears. Using a rubber spatula so you do not damage the mat, scrape the pear mixture back into your food processor or blender. Add the butter. Blend until heavenly smooth and glistening.
There is a reason to add the butter now, as opposed to before. Heat damages oil. Time damages oil. Light also damages oil. All oil. Butter is oil. If that butter melts and cooks with the pears all together, it will taste good hot, and pretty blah when it gets cold. This way, well, you will see. It’s also great served with sharp cheese and crackers, some dried figs, that sort of thing. On toast. You see Membrillo in the stores, the price they charge for it is shocking.