Ever wonder what they call the middle bun on a Big Mac? There’s a lot to the Big Mac and McDonald’s newest menu item, the Chicken Big Mac. The traditional Big Mac is a burger with two all-beef patties, special sauce, pickle, lettuce, onion and cheese. The new version of the sandwich swaps out the beef for tempura-breaded chicken patties. That’s a long list of ingredients to put on a sandwich, but it is actually designed to be assembled in less than 25 seconds once the chicken comes out of the fryer. READ MORE: What It Means If You See A McDonald’s With Black ArchesIt’s probably easy to do if y…