However, Burger King in the US publishes a disclaimer which states: "Burger King Corporation makes no claim that the BK Veggie Burger or any other of its products meets the requirements of a vegan or vegetarian diet. Burger King's new BK Single Stacker replaces the Buck Double on the BK Value Menu for $1. I guess a slice of bacon is cheaper than an extra beef patty. Each Stacker comes with a single beef patty, a slice of cheese, two half slices of bacon and BK Stacker sauce. Bk Stackers are also available with a. The Double Stacker is back at Burger King bringing with it the return of the mysterious Stacker sauce. The Double Stacker was originally introduced back in as part of the now-discontinued BK Stacker line, which featured a burger with a choice of one, two, or four patties (the Single, Double.