Cut the lettuce into bite-sized pieces. In a bowl, mix the warm cooked rice with potato starch, then divide into 4 portions. Wrap each portion in plastic wrap, shape into a round bun, and gently press to form rice buns.

Heat half of the sesame oil (1 tbsp) in a frying pan over medium heat. Place the rice buns in the pan and cook until both sides are golden brown. Remove and set aside.

In the same pan, heat the remaining sesame oil (1 tbsp) and cook the sliced beef over medium heat until browned. Add the seasonings (sake, sugar, soy sauce, and doubanjiang) and stir-fry until the liquid evaporates.