What an incredible brood pattern. You have and excellent queen and an excellent hive. I agree, it's a queen cup. First few times I saw those, I freaked out thinking they were trying to build a queen cell (which would mean either faulty queen or no queen, both heart stoppers for me). I called my bee mentor and he explained what everyone else has shared, it's a queen cup. It's preparation for an egg to produce a queen should they need it. My mentor called them "threat cells" to the queen, i.e. get to work or we'll replace you.
Enjoy your bees, they are fabulous. Wish I could say that all of my queens produce brood patterns like that.
good luck.
8-)