Wet your hands slightly in a dish of water. Take 3-4 Tbsps of the rice in your hands and form into a ball, as if your were making a snowball, or into a triangle by cupping your hands into a V shape. Pack the rice to form a solid ball or triangle. Form the remainder of the rice into balls or triangles.

Poke a hole into the center of each rice ball or triangle with your index finger. Place a small piece of the umeboshi plum in each hole. Press the balls with your hands to close the holes.

Roll each ball or triangle in the roasted sesame seeds until completely coated.