Hamburger buns are soft rolls, usually already split horizontally to accommodate a burger patty. The hamburger bun was invented in 1916 by a fry cook named Walter Anderson, who co-founded White Castle in 1921. A soft, round yeast roll 3½ to 4 inches in diameter, made to fit the size of a hamburger. It may be made with regular or whole-wheat flour and variously topped with flavorings such as sesame seed, poppy seed or toasted chopped onion.