Soak the beans overnight in more than enough water to cover.
In the morning, drain the beans and add to the slow cooker.
In a large skillet set to medium high heat, add the cooking oil. Add the thick sliced onion rounds and brown them on both sides until they are really toasty. Add the onions to the slow cooker.
Add the cayenne pepper, black pepper, garlic cloves, chicken bouillon cubes, and water. Stir.
Cover and cook on low for 9 hours.
When the cooking time is done, take out at least half of the juices from the beans, and set aside, you can use this if too much liquid was taken out.
You can either put the beans in a blender, mash by hand with a potato masher, or use a emulsion blender like I did in my video. If your beans seem too stiff, add some of the reserved liquid. Add the butter into the mashed beans.
I did not need to add any salt for my beans, but if you think they need a bit of salt this is the time to add it to taste.
Top with the optional cheese and cilantro, if desired.