The following are obligated in tithes but exempt from challah: rice, millet, poppy seeds, sesame, legumes and grain that will yield less than a kav and a quarter of flour (a little over a half-gallon). Challah need not be taken from honey cakes, fried dough, or from medumah (terumah that fell into other produce, limiting its consumption to kohanim).
If a dough started out as cake dough and ended up as cake dough, it is exempt from challah. If it started out as regular (bread) dough and ended up as cake dough, or started out as cake dough and ended up as regular (bread) dough, it is obligated in challah. Cakes made from bread flour are obligated in challah.