Downstairs with Kuroshitsuji are the extras that occasionally appear at the end of the volumes. They are drawn by Yana Toboso and commonly depict what goes into making Kuroshitsuji. They are numbered as volumes, but the number is not necessarily the same number as the volume it appears in due to them not appearing in every volume.
Volume 2: Found in Volume 2. She talks about how the series got its name, the strange names her editor thought of, how the Viscount of Druitt got the nickname "koshaku" (insolent), the research she does for the series, and the drama CD.
Volume 3: Found in Volume 5. She talks about going to a count's home to take pictures for the "Phantom Night" (a Kuroshitsuji event) and her editor, Editor K, appearing in all of them. She also mentions the announcement for the anime adaptation.
Volume 6: Found in Volume 13. She talks about her experiences as a beginner mangaka and the mistakes she made when she was learning.
Volume 7: Found in Volume 17. She talks about doing research about cricket and about her trip to the Japan Cricket Association to learn more about the sport.
Volume 8: Found in Volume 21. She reveals the nicknames she and her assistants came up with for some of the characters in the series. She mentions that characters are called by nicknames until everyone remembers their real names.