Pour rice directly into a heavy bottomed pot or rice cooker bowl. Rinse with cold, running water, scrubbing grains gently with hands. Drain off water and repeat multiple times until the water runs almost clear. If time permits, allow rice to soak for at least 30 minutes.
If using a rice cooker, follow rice cooker’s instructions to prepare.
If cooking rice on the stove, drain rice thoroughly then add 2 cups of water. Cover pot with lid and bring water to a boil over medium-high heat. As soon as water boils, turn heat to a very low simmer.
Keep pot covered until water is completely absorbed, typically 12–15 minutes. Turn heat off and leave pot undisturbed for 10 minutes, then fluff the rice gently with a wooden spoon or rice paddle.
Create tare by mixing soy sauce, mirin, sesame oil and dashi powder in a small bowl. Whisk to combine. Drizzle over freshly steamed rice, and swiftly combine by folding and turning the rice gently with a spatula or rice paddle until the color is uniform. Be careful not to churn or crush the grains of rice.
To form onigiri by hand, empty hot rice into a large bowl or platter and separate into 6–8 equal piles.
Wet your hands with water and scoop a pile of rice into your non-dominant palm.
Gently compress the rice into a triangle by cupping the rice with your dominant hand into a pyramid shape. Flip the rice on the surface of your non-dominant palm multiple times, squeezing gently without breaking or crushing the grains of rice. Use just enough force to prevent the onigiri from falling apart as you smooth it into a flat-surfaced, evenly proportioned triangle.
Heat a well-oiled grill to medium-high heat. Place onigiri directly on the grates and grill undisturbed for 8–10 minutes until crisp and well browned. Flip, then brown the opposite side an additional 8–10 minutes. Wrap in shiso leaf and serve. Makes 6–8 onigiri.
*Many Asian grocery stores sell inexpensive plastic onigiri molds to easily form uniform, triangular shapes. If using, fill still-hot rice to the top rim of the mold. Gently press, using just enough force to prevent onigiri from falling apart, and release.