Use the measuring cup and water lines on the cooker bowl to cook 1 rice cooker cup of rice.
If you're doing it on the stove, wash the rice in a few changes of water until it runs mostly clear. Drain it in a sieve. Add the rice to a tall pot along with 1 US cup of water. Cover the pot with a lid, and then bring the water to a boil over high heat. As soon as it's boiling, turn the heat down as low as it will go. Set a timer for 13 minutes. After the timer goes off, turn the heat off and let the rice continue steaming for another 10-15 minutes. Do not open the lid at any point during the cooking or steaming process.
Once it's done steaming, pour the soy sauce evenly over the rice and use a folding and cutting motion with a spatula or rice paddle to evenly distribute the soy sauce until the rice is uniform in color.
Level off the top of the rice and then divide it into thirds like a pizza.
If you're shaping the onigiri by hand, click this link for specific instructions. If you're using an onigiri press, just get it wet and stuff each mold with a segment of rice. Press the onigiri together more than you normally would for onigiri so that it holds its shape.
Put the onigiri on a parchment-lined tray. Dust each onigiri with a thin layer of potato starch by passing it through a fine-mesh sieve like a tea strainer. Flip the onigiri over and dust the other side. Try not to get starch onto the sides of the onigiri.
Put a non-stick frying pan over medium-low heat and when it is moderately hot, add about half of the oil and arrange the onigiri, so they're sitting on a small pool of oil.
Fry undisturbed until they're golden brown and crisp on one side (about 7-10 minutes).
Flip them over, and then add a bit more oil to fry the second side until crisp and browned.
Finish the Yaki Onigiri off by melting some butter directly onto each browned surface of the onigiri and then roll them on their 3 sides to cook any stray starch.
Serve the grilled rice balls wrapped in shiso or nori.