Instead of a hamburger bun, the Breakfast Stack includes three English muffin halves. The bottom half is basically a sausage muffin with a sausage patty, ketchup, and cheese, while the upper half is your standard bacon and egg breakfast sandwich with three strips of bacon, a poached egg, mayo, and cheese. The two sections are separated by a middle English muffin.
Should you find that the third bun is unnecessary, they also have something on their regular menu called the King Muffin. It features two sausage patties, two slices of cheese, bacon, egg, and ketchup on a toasted English muffin.