How is this not an option here yet? Staring next Wednesday, McDonald's will start selling a CHICKEN BIG MAC with chicken instead of burgers, but only in the U.K.
It's just like a regular Big Mac, but with two fried chicken patties. And they're just testing it, so it'll only be on the U.K. menu until March 15th. A few countries in the Middle East already have it on their permanent menu though. So it's not a totally new thing.
It's not clear if they plan to test it here anytime soon. So if you want one, the best you can do is hack it. Just order a Big Mac, but ask for McChicken patties instead of burgers. They might charge you more though.