May one eat a cake/challah/cookie shaped as a number or letter on Shabbos? How about shaped as a picture?
It is proper to be stringent not to eat and break bakery items that have on them letters or pictures on Shabbos and Yom Tov.
This applies even if the shape is carved out from the dough itself.
However if the entire challah is the shape (besides shapes of letters or numbers) of a design one may eat it. An example would be be eating a key shaped challah on the Shabbos after Pesach.
See Shulchan Aruch Admor Hazaken (Orach Chayim 340:4; 343:10; 458:8; 519:6).
See also Ketzos Hashulchan 144 (Badei Hashulchan 3) who is somewhat lenient. Hashmatos to ketzos hashulchan where he retracts his opinion. Answer of the Rebbe in Yismach Yisroel (Rabbi B Kohen) 25.
See also Shabbos Kehalacha 20:73-75 footnote 185 and Biurim אות לב.
ודשו בזה רבים בקובצי הערות בכ״מ ועוד.
ולהעיר, משו״ע אדה״ז סתקי״ט ס״ו שמשום מה העלימו כמה עין מזה.