Basically any baked goods are great to give to hens in moderation!
About once every other day or so keeps them from getting too full on bread, cake, bagels, etc. One small cake (depending on how big it is or how many chickens you have) could last for quite a while. I bet your chickens will enjoy the cake!
What I do is I soak their regular food, put a think layer in the cupcake holder, crush some apple pieces and stick them on top of the layer, then put another soaked layer on top of the crushed apple, then put some racehorse oats on top, TA-DA!