The Beis Yosef Orach chaim 297 quotes an opinion which the Magein Avraham 297,1 supports, that one can make a Brocho of Borei minei Besamim on smelling a Regular bun that emits a nice smell when its hot - Though the Aruch Hashulchan Orach Chaim 297,5 mentions that this is not the practiced Minhag:
יש אומרים דאפילו על פת חם מבדילין (עיין ב"י ומג"א), ולא שמענו מי שעשה כן
Since the Cinamon ontop the bun emits a nice natural smell and it is a spice, even those that argue bread alone is not suficient a cinamon bun is fit for saying Borei minei Besamim (the Mishna Brura 297,1 1 says we make Borei Minei Besamim even on products from bark or the leaves) even though the cinamon bun is for food purposes, though ideally have a set aside uniquely for smelling Besamim (Mishna Brura 297,4 10).