Prepare the sauce. Mix together the soy sauce, mirin, and sugar. Set aside.
In a large mixing bowl, mix together the hot cooked rice, corn, and salt. Fold gently to avoid breaking the grains.
Using your hands or an onigiri mold, shape the rice-corn mixture into 6 onigiri.
If using your hands, wet them first with water to prevent the rice from sticking. Scoop a handful of rice into your hands and begin packing and pressing firmly, as if making a snowball. Continue packing the rice until it is firm and evenly shaped.
In a cast iron skillet or nonstick frying pan, melt the butter over medium-high heat.
Place the onigiri flat into the skillet and let the butter sizzle around the rice for about 4-5 minutes, until side is lightly crisp. Flip the onigiri over and repeat the other side.
Using a basting brush, brush on some of the soy glaze onto the onigiri. Flip the onigiri over one more time to char the basted side. Repeat the basting and crisping process with the other side.
Cook the onigiri until lightly browned and the outside is crunchy. Remove from heat.
If desired, wrap the onigiri with nori strips and serve immediately.