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 160 grams Japanese short-grain rice in a few changes of water until it runs mostly clear. Drain it in a sieve. Add it to a tall pot along with 1 cup 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 it continue steaming for another 10-15 minutes. Do not open the lid at any point during the cooking or steaming process.
160 grams Japanese short-grain rice, 1 cup water
Once it's done steaming, pour the 2 tablespoon soy sauce evenly over the rice and use a folding and cutting motion with a spatula or paddle to evenly distribute the soy sauce until it is uniform in color.
2 tablespoon soy sauce
Level off the top of the mixture and then divide it into thirds like a pizza.
If you're shaping the yaki onigiri by hand, click this link for specific instructions. If you're using an onigiri mold, just get it wet and stuff each mold with a segment of rice. Press the onigiri mold together more than you normally would so that it holds its shape. Put the onigiri on a parchment-lined or plastic wrap-lined tray. Dust each onigiri with a thin coating of the 1 teaspoon 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.
1 teaspoon potato starch
Put a non-stick frying pan over medium-low heat and when it is moderately hot, add about half of the 1 tablespoon vegetable oil and arrange the onigiri so they're sitting on a small pool of oil.
1 tablespoon vegetable 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 of the 1 tablespoon cultured unsalted butter directly onto each browned surface of the onigiri and then roll them on their 3 sides to cook any stray starch.
1 tablespoon cultured unsalted butter
Serve the fried rice balls wrapped in Shiso leaves or Nori sheets
Shiso leaves, Nori sheets