Place bacon into a large cast iron skillet over medium heat and cook until fully browned, 7 to 10 minutes. Remove bacon to a paper towel-lined plate, but leave the bacon grease. Chop bacon when cool enough to handle.
Add 1/4 cup butter and black garlic clove to the bacon grease. Mash the black garlic thoroughly, then cook and stir until fragrant, 1 to 2 minutes. Add Brussels sprouts and remaining butter; season with salt and pepper. Cook, stirring only a couple of times, until sprouts are browned and tender, 5 to 7 minutes.
Remove from heat and add bacon and Parmesan cheese; stir until heated through and cheese is melted. Place into serving dish and cover with a lid if not serving immediately.