In a pan over medium heat, combine the leftover rice with the hoisin sauce, rice vinegar and nori sheet.
In a small saucer, pack 1 to 2 tablespoons of rice, and then make a divot in the center. Place the sweet potato in the centre of the rice, and then layer 1 to 2 more tablespoons of the rice over top
Wet your hands, then transfer the rice “ball” into your palms. Pack the rice around the sweet potato until a firm tight ball is formed. Repeat until all of the rice and sweet potato is combined.
Heat a pan over medium, and grill the rice balls until slightly toasted on each side. Pour the sesame seeds on a plate, and roll each of the rice balls in the seeds until slightly covered.