American fast food hamburger chain Burger King has expanded its menu with the addition of the new toasted Sourdough bun range at participating restaurants nationwide.

The Sourdough bun range includes two offerings, the Sourdough King and the Breakfast Sourdough King, which feature the ingredients of Burger King restaurants’ flame-grilled burgers but with a toasted sourdough bun.

The Sourdough King is prepared using a half pound pre-cooked and flame-grilled beef patty, topped with crispy bacon, melted American cheese, sliced onions and creamy sauce.

The Breakfast Sourdough King is made with sausage, smoked bacon and sweet black forest ham. It is served with warm eggs and two helpings of melted American cheese on a toasted sourdough bun.

The Sourdough King is available for $5.99 and the Breakfast Sourdough King collection for $3.69 at participating restaurants for a limited time.

The chain is also inviting guests to participate in a voting poll on Burger King’s social channels to help decide whether Sourdough King is a burger or a sandwich.

Burger King, which was founded in 1954 and owned by Restaurant Brands International, now operates more than 15,000 locations in nearly 100 countries and US territories.

With more than $27bn in system-wide sales, Restaurant Brands International is a quick service restaurant company that operates more than 23,000 restaurants in over 100 countries and US territories.