Place dry beans in the bottom of slow-cooker. Pour water over top. Add the bouillon cubes, garlic powder, and salt. Place the onion half right on top of the beans. Cover and cook on low for 8 hours, or until beans are very soft and fall apart when mashed (if you undercook your beans, they will be grainy and difficult to mash).
Remove the onion and drain the beans, reserving the cooking liquid. Return the beans to the pot, add the melted butter and the milk. Blend with a hand immersion blender until creamy and smooth, adding reserved juices from the pot until desired consistency is achieved (more milk can also be added if you like them creamy). Note: the beans firm up as they cool, so make them looser than you think you need to. Serve promptly. Store leftovers covered in the refrigerator.